Can not send large attachments

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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 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. (In 3.x versions the setting is accessible only through the Config Editor - Tools->Options->Advanced->General->Config Editor.)
  • Disable any program such as an anti-virus or spyware scanner from scanning outgoing messages. Its recommended that you don't scan outgoing messages anyways because it frequently causes interoperability problems, and if the recipient doesn't have their own anti-virus program they have bigger things to worry about than your message.
  • Your TCP-IP stack might need tweaking. However, be careful changing MTU or RWIN, if you don't know what you're doing you can cripple TCP-IP. If you're not used to tweaking your operating system ask for help in your ISPs forum , or the MozillaZine Tech forum before you do anything.
  • Lower the Maximum Transmission Unit (MTU) used by TCP-IP . This decreases performance (throughput) but should create a more robust TCP-IP connection. Don't set a MTU smaller than 1,400. If you have a router its MTU should be set to the same value as your PC.
  • Vista has a Receive Window Auto-Tuning feature that tweaks RWIN that can fail when used with older routers (especially ADSL routers) and some firewalls. It can usually be fixed by updating the routers firmware. You can also disable Receive Window Auto-Tuning.
  • Try a TCP Tweak tool [1] [2] [3], picking a safe setting rather than trying for the fastest connection speed.
  • Try a different SMTP server if one is available.
  • If you're using POP before SMTP authentication try increasing mail.pop3_response_timeout.
  • You might have a hardware problem that only shows up when sending large amounts of data. This sounds like a lame excuse because everything else is working okay, but most other programs use the network differently.
  • Reboot the router (if you have one) and the modem. Usually its easiest to do this by powering them off, and then turning the power back on.
  • Replace the cable between your PC and modem if you have a spare. Usually this is either a Ethernet or a USB cable.
  • Check if the firmware on your router (especially if its a wireless router) and modem is up-to-date.

Its also possible the problem has nothing to do with the size of the attachment. Try the following:

  • 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.
  • Check that the attachments filename doesn't contain characters that might cause a problem by renaming it beforehand to use just 7bit ASCII letters.
  • See if the problem only occurs for a specific file name extension. A number of users have had problems sending .doc attachments and fixed the problem by exiting Thunderbird and deleting the MimeTypes.rdf file in the profile. Its not clear why this works since its normally used to associate a MIME type with another program, and Thunderbird normally doesn't call another program when it sends a message to the SMTP server. If you do this it will reset what is used to view an attachment with a specific file type to the default values.


If you can't send a message with a large attachment because it exceeded your email providers limits consider transferring the attachment using one of the following services instead. Some of them will also send a message with the files URL to the recipient for you. Read their terms and conditions, and privacy statement first.

You could also use a file splitting utility to break the file into multiple pieces that you send in separate messages. Many of them don't require the recipient to use the same utility to reassemble the file, they'll create either a batch file or a tiny .exe that you can include as an attachment that will do that.

See also

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