Send email attachments over SMTP by structuring your message in Multipurpose Internet Mail Extensions (MIME) format.
Sending attachments over SMTP requires the email body to follow the MIME format. MIME defines how to package multiple content parts—body text, images, files—into a single email message that any standard mail client can parse.
MIME basics
MIME is an Internet standard that extends the format of email messages.
For more information, see RFC 2045.
How attachments are encoded
Email protocols transmit text, not binary data. To carry binary files, MIME uses Base64 encoding to convert raw bytes into a safe ASCII representation. This adds roughly 33–37% to the original file size, so a 10 MB file becomes approximately 13–14 MB after encoding.
A MIME message with attachments uses multipart/mixed as the top-level content type. Each part is separated by a unique boundary string and carries its own headers. An attachment part must include:
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Header |
Description |
Example |
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The file's MIME type and filename |
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Set to |
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Set to |
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The following example shows a minimal complete MIME message with one attachment:
From: sender@example.com
To: recipient@example.com
Subject: Monthly report
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary123"
--boundary123
Content-Type: text/plain; charset="UTF-8"
Please find this month's report attached.
--boundary123
Content-Type: application/pdf; name="report.pdf"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="report.pdf"
JVBERi0xLjQKJ...
--boundary123--If this structure is malformed—for example, a missing boundary or incorrect header—email clients may display corrupted files or silently drop attachments.
The total size of an email with attachments cannot exceed 15 MB. An email can contain a maximum of 100 attachments.
The 15 MB limit applies to the total size of the sent email, which includes the email header, email body, and attachments. Because Base64 encoding increases the file size, the final email can be more than 1.5 times the size of the original attachments. Therefore, keep the total size of your original attachment files to 8 MB or less. To send larger files, add hyperlinks to them in the email body.
Code examples
The following pages provide working SMTP code examples in several languages. Each example constructs a multipart/mixed MIME message and sends it over SMTP.
For sample code, see: