Encrypting email attachments: How to do it securely

Whether contracts, quotations or internal documents – email attachments often contain confidential information. What many people underestimate is that standard emails are not inherently secure. If you take data protection seriously – or are legally obliged to do so (for example, under the GDPR) – you should never send attachments unencrypted.

In this article, we explain why encrypted attachments are important, outline the available methods along with their advantages and disadvantages, and show you how to encrypt an email attachment securely and easily using FTAPI.

Why you should encrypt an email attachment

Email remains the most vital communication tool in daily business life: According to Bitkom (January 2026), an average of 53 emails arrive in business inboxes in Germany every day—up from 40 emails just two years ago. Furthermore, one in seven employees (14%) receives 100 or more work-related emails daily.

Sending an unprotected email is like sending a postcard – anyone with access along the way or at the destination can read it. Sensitive content in attachments – such as contracts, quotations or personal data – will therefore arrive unencrypted in inboxes, assuming it even reaches the correct recipient and is not intercepted en route.

What’s more, email remains the preferred attack vector for cybercriminals. According to Bitkom, numerous cyberattacks on companies are initiated via email – and in 63 percent of data theft cases, general communication data such as emails is affected.

This makes unencrypted attachments particularly risky – whether due to mistyped addresses, unsecured Wi-Fi networks or compromised accounts. Even password-protected ZIP or PDF files offer genuine protection only if the password is sent separately and securely via another channel (for example, by phone, not just in a separate email) – something that rarely happens in practice.

Encryption is also a legal requirement: the GDPR stipulates that personal data must be protected through appropriate technical measures – and this includes attachments containing such data. Companies that neglect this risk fines, reputational damage and losing control over their data.

The good news: Secure communication no longer has to be complicated. There are now several solutions that allow you to send attachments in an encrypted, traceable way – without technical hurdles.

Let’s take a closer look at what that can look like.

How can email attachments be encrypted?

The main purpose of email encryption is to prevent unauthorised access to the data contained in an email. This data may include email addresses, the message body itself (see our dedicated article on email data protection for more details) or attachments. Attachments often contain particularly sensitive information and are frequently overlooked when it comes to protection.

To secure attachments in emails, there are various encryption methods and essentially two types of protection: transport encryption (e.g. TLS) and end-to-end encryption (e.g. PGP and S/MIME).

The following sections will explain when each method is appropriate and outline their respective advantages and disadvantages.

Transport encryption (TLS)

Transport encryption, also known as point-to-point encryption, uses TLS (Transport Layer Security) and is the standard with most email providers. It protects only the transmission path – that is, the connection from one email server to another. The contents of the email – including both the message text and attachments – remain unencrypted.

Example: An email with an attachment is sent from Server A to Server B. TLS ensures that no one can intercept the message while it is in transit. However, once the email is stored on the sender’s server, temporarily cached at network nodes, or sitting in the recipient’s inbox, it exists in plain text and is therefore vulnerable – for example, in the case of a compromised mailbox or man-in-the-middle attack.

TLS, therefore, serves as a basic level of protection. While it is suitable for routine coordination, organisational coordination (e.g. appointments), or general service information, it is insufficient for confidential content.

Protecting email attachments with a password (symmetric encryption)

Another option is to encrypt only the attachment – for example as a ZIP archive or a password-protected PDF. Both parties must know the same password, which therefore also has to be transmitted securely.

The problem: if you send both the file and the password by email, your data is not truly protected. Password-protected attachments offer only limited security, much like TLS. They are only effective if the password is transmitted securely (e.g. by phone or via a messaging app) and is not reused. In practice, this is rarely the case.

End-to-end encryption

With end-to-end encryption, the email content, including attachments, is encrypted directly in the sender’s email client and only decrypted on the recipient’s device. The data remains encrypted throughout transmission and while stored on servers – third parties have no access.

This principle can be implemented in different ways:

• Symmetric encryption uses the same password for encryption and decryption – for example with password-protected PDFs. The sender and recipient must exchange the password securely in advance.

• Asymmetric encryption uses a key pair: a public key for encryption and a private key for decryption, which is not shared.

The best-known methods are:

S/MIME (Secure/Multipurpose Internet Mail Extensions)

S/MIME is based on digital certificates issued by a recognised certification authority and then used as keys. Both sender and recipient each have a key pair:

• The public key is shared and used for encryption.

• The private key remains with the recipient and is used for decryption.

Advantages:

• Also supports digital signatures

• Suitable for long-term, secured communication relationships

Disadvantages:

• Setting up and managing certificates the classic way is time-consuming

• Both parties need valid certificates

• Normally not very flexible when communicating with external recipients

OpenPGP / PGP (Pretty Good Privacy)

PGP (also OpenPGP) is an open standard without central certification authorities. Users generate their own key pairs and exchange public keys manually or publish them on key servers.

Advantages:

• Strong cryptographic security

• No dependency on certification authorities

• Open, well-established standard

Disadvantages:

• Technically complex to use

• Not integrated by default in email clients

• High training and administrative cost

• Limited suitability in corporate environments

Platform-based end-to-end encryption (e.g. with FTAPI)

Platform solutions like FTAPI bridge the gap between robust protection and ease of use: they utilise technically established standards and automate the encryption process in the background. Users benefit from automated end-to-end encryption without manual key exchanges and without the constant burden of deciding which encryption method is correct. In this setup, FTAPI acts as a central hub for all email communication.

Advantages:

• Certificate-based encryption is possible – without the need for manual certificate management

• Seamless integration into Microsoft Outlook is available

• Easy for both internal and external recipients to use

• Supports very large files (via the Outlook Add-in)

• Automatic selection of the appropriate encryption method (via mail-flow integration)

• GDPR-compliant and aligned with BSI guidance

Disadvantages:

• Attachments are not stored directly in the email

• Link-based delivery may feel unfamiliar to some recipients

Summary: email encryption in simple terms

• TLS protects only the route an email takes – not its contents. While this basic protection is sufficient for routine emails, it is inadequate for sensitive personal data.

• Password-protected attachments are secure only if the password is sent separately via another channel (not just in a separate email).

• S/MIME and PGP provide full protection (both path and content) but are typically technically demanding to manage manually.

• Platforms such as FTAPI offer a pragmatic, secure approach by automating the encryption process. The system automatically selects the most secure method without any user effort – whether that is S/MIME or delivery via an encrypted link – ensuring attachments are always protected and accessible only to authorised recipients.

The next section provides a step-by-step guide to encrypting email attachments with FTAPI.

How to encrypt an email attachment with FTAPI

With FTAPI, you have two fundamentally different options for securely encrypting email attachments – depending on your specific use case and recipient structure:

1. Platform as a user tool: Encryption takes place directly within the email programme, thanks to Outlook integration. When sending, employees decide whether the message and its attachments should be transmitted in encrypted form.

2. Mail-flow integration: Encryption is implemented as a system component within the mail-flow. All outgoing messages are automatically checked for sensitive content and encrypted where necessary. Employees do not need to take any extra steps; they write and send messages as normal within their email programme.

We will look at how both options function in more detail in the next section.

Option 1: Standard delivery via FTAPI SecuMails (user tool)

With FTAPI SecuMails, you can send attachments securely and in compliance with the GDPR, directly from your email inbox. The solution can be used via the web or Outlook and allows you to transfer attachments of any size in encrypted form – to both internal and external recipients, without requiring them to have their own infrastructure. The file is uploaded in encrypted form and provided via a secure link accessible only to authorised recipients.

How sending with SecuMails works:

1. Write your email as usual: Enter the recipient’s address, subject and your message. In Outlook, use the standard email window. In the web application, click on “New delivery” and fill in the relevant fields.

2. Add attachments: Attach the files you wish to send securely. In Outlook, you add files as usual. In the browser, you can upload them via drag and drop or by clicking the “Attach files” button.

3. Choose security level and expiry date (optional): Decide on the security level for your files and how long they should be available for download. These settings can also be centrally managed within the organisation.

4. Insert a download button (optional, Outlook only): When sending via Outlook, you can insert a “Download button” for your attachments into your email. This can be placed manually or automatically added at the end of your delivery (above the signature).

5. Send your file securely: Click “Send with FTAPI” in the menu bar to securely transmit your email and attachments.

The recipient will then receive an email with a download link in their regular inbox and can safely retrieve your files. Depending on the selected security level, authentication may be required before the download.

Option 2: Automatic encryption (mail-flow integration)

Organisations can secure their email communication with FTAPI using certificates – for example, via the established S/MIME standard – in a fully automated and integrated way. To achieve this, FTAPI integrates directly into the email flow:

• Compose and send emails as usual: You write your emails as you normally would in your email programme. Subject line, message and attachments are entered as usual – no additional tool or special delivery method is required.

• Encrypt correctly – automatically: FTAPI acts as the central hub. The platform integrates into your existing infrastructure and decides how to encrypt the data. Administrators define the protection rules once, and FTAPI applies them automatically to every email. For instance, if a recipient supports S/MIME, FTAPI automatically encrypts using that standard. If not, FTAPI’s own SecuMails encryption takes over seamlessly. This ensures every message is optimally protected without extra effort, regardless of the recipient’s IT setup.

• Manage certificates: The certificates required for encryption (e.g. S/MIME) are managed centrally via FTAPI, including their issuance and automatic renewal.

• Archive emails: If required, encrypted emails can be forwarded in a decrypted format to existing archiving systems. This ensures that legal retention requirements are reliably met, even for encrypted communication.

Furthermore, every transmission is logged. This works with any email programme without the need for additional add-ins. Hosted in Germany, the platform provides reliable protection in accordance with GDPR.

FTAPI thus provides a central solution for a diverse range of use cases – from spontaneous individual messages to secure, structured communication with partners and public authorities.

Conclusion: Encrypted attachments should become standard practice

Email attachments often contain particularly sensitive information. Without adequate protection, they can easily end up in the wrong hands – whether through a mistyped recipient address, an unsecured Wi-Fi network or a compromised mailbox. Anyone wishing to comply with the GDPR and avoid data breaches should always encrypt attachments.

The most suitable method depends on the use case. Platform-based solutions like FTAPI, which automatically select the appropriate encryption and feature integrated certificate-based options (e.g. S/MIME), offer the greatest flexibility. Crucially, encryption should not be a complication – it should be a fundamental part of secure corporate communication. True compliance is achieved when email encryption is no longer something you have to actively "do", but something that simply happens.

By automating encryption, you make secure data exchange as natural as sending a standard email, transforming security into a driver of efficiency