Can not send large attachments

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

Sometimes you can send a small message successfully, but if you try to send one with a large attachment you get a error message like:

Sending of message failed.

The message could not be sent because connecting to SMTP server mail.adelphia.net failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server settings is correct and try again, or else contact your network administrator.

Try the following:

• Find out the maximum attachment size the SMTP server supports. Thunderbird doesn't impose any limit. The size of a binary file is increased by about a third when you send it as an attachment due to the base64 encoding. The best way to verify that you're not running into a size limit is to see if you have the same problem using another email client such as Outlook Express.

• Increase mailnews.tcptimeout from its default value of 60 seconds to 300 seconds using Tools -> Options -> Advanced -> General -> Connection timeout. This frequently solves the problem, especially when you have a wireless connection. This preference is only available in 1.5 or later.

• If your anti-virus program scans outgoing messages disable that feature. Its recommended that you don't scan outgoing messages anyways because it frequently causes interoperability problems, and if the recipient doesn't have thier own anti-virus program they have bigger things to worry about than your message.

• Try a different SMTP server if one is available.

• Send a message with a small attachment with the same file extension to verify your SMTP server is not filtering based on the file extension. You might also try sending the attachment as a .ZIP file.

• If you're using POP before SMTP authentication try increasing mail.pop3_response_timeout.

See also

• Can not send messages
• Modify Thunderbird settings