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Sending limits

Check these recipient, message-size, and attachment limits before you send email through the API or SMTP relay.

Maximum recipients

Each email can have a maximum of 50 recipients combined across the to, cc, and bcc fields. This limit applies to the REST API, including each email in a batch, and to the SMTP relay.

This limit aligns with transactional email best practices. Transactional emails (receipts, notifications, password resets) are typically sent to a small number of recipients. If you need to send to larger lists, consider:

• Using the batch API for sending the same email to multiple recipients
• Splitting large recipient lists into multiple API calls
• Using broadcast routes for marketing or newsletter-style emails

Email size limit

The maximum size for an email message (including all attachments) is 25MB. This limit includes:

• The email body (HTML and plain text)
• All attachments (base64 encoded)
• Headers and metadata

Over SMTP, an email over the size limit gets a 552 reply. See SMTP response codes.

Request size limit

A request to the API can be at most 25MB. This counts the whole JSON request body, including base64-encoded attachments, and applies to the single send and the batch endpoints.

A larger request is rejected with 413 Request Entity Too Large, and nothing in it is sent. For a batch, this means none of its emails are sent, even when the batch has 500 emails or fewer. Split large batches into several requests that each stay under 25MB.

Header formatting

Send the subject, display names, custom header values and attachment filenames as plain text. Lettermint encodes them for the email standards when it builds the message. Text that is already ASCII goes out unchanged, and only text with other characters is encoded. Spam filters such as Rspamd add points for headers that are encoded without a reason, so leave the encoding to Lettermint.

Lettermint repairs small formatting errors in headers before it sends the email:

• Line breaks and tabs become spaces, and other control characters are removed.
• In the subject and display names, text you encoded yourself, such as =?UTF-8?Q?Hello?=, is decoded and encoded again only if it needs to be. Other custom header values keep their content, because they can hold data such as JSON.
• A header name with spaces or a trailing colon is fixed, so X Custom Header: becomes X-Custom-Header.
• References and In-Reply-To get angle brackets around each message ID. Values that are not message IDs are removed.
• List-Unsubscribe and the other list URL headers get angle brackets around each URL, and a bare email address becomes a mailto: URL.
• A display name longer than 255 characters is shortened.

The API rejects a header name that is still not valid after this, such as one with a colon or non-ASCII characters inside it. It also rejects a List-Unsubscribe or other list URL header that contains no usable URL. Lettermint removes any other header it cannot repair, including such headers in SMTP messages.

Blocked attachments

For security reasons, certain file types are blocked from being sent as attachments. These restrictions help protect recipients from potentially malicious files.

Over SMTP, an email with a blocked attachment gets a 550 5.7.1 reply that names the file extension or content type. See SMTP response codes.

Blocked file types

The following table lists all blocked file extensions and their associated MIME types, organized by category:

Executable files & applications

Script files

System & configuration files

Microsoft office & access

Web & browser files

Help & documentation

Shell & shortcuts

Installer & package files

Visual studio & development

Media files

FoxPro files

Temporary files

Miscellaneous

Additional MIME type restrictions

The blocked-file policy reports these MIME types:

• application/asp
• application/gadget
• application/hta
• application/java-archive
• application/javascript
• application/octet-stream
• application/octet-stream-exe
• application/octet-stream-sys
• application/vnd.ms-access
• application/vnd.ms-htmlhelp
• application/vnd.ms-vsmacros
• application/x-asp
• application/x-bat
• application/x-chm
• application/x-cmd
• application/x-crt
• application/x-csh
• application/x-executable
• application/x-gadget
• application/x-javascript
• application/x-ms-application
• application/x-ms-installer
• application/x-ms-shortcut
• application/x-ms-wmz
• application/x-msaccess
• application/x-msbinder
• application/x-msdos-program
• application/x-msdownload
• application/x-msi
• application/x-msmediaview
• application/x-msmetafile
• application/x-mspublisher
• application/x-msscript
• application/x-powershell
• application/x-reg
• application/x-sh
• application/x-shellscript
• application/x-sys
• application/x-vb
• application/x-vbs
• text/asp
• text/javascript
• text/jscript
• text/scriptlet
• text/vbscript
• text/x-asp
• text/x-powershell
• text/x-script

Why these restrictions?

These file types are blocked because they can:

• Execute code automatically when opened
• Exploit vulnerabilities in email clients or operating systems
• Bypass security measures through social engineering
• Contain macros or scripts that could harm the recipient's system
• Be used to spread malware or ransomware