If Thunderbird is not forwarding your email attachments, it's likely not a server problem. Thunderbird by default does not send attachments as separate files. Instead, attachments are sent inline with your message. When you send an email with an attachment, its contents are displayed at the bottom of the message.
When an attachment is displayed within the body of an email, it may not convert properly. For example, plain text attachments with encoded data won't convert to text very well. The recipient of your email will see meaningless, jumbled text and symbols at the bottom of the message.
One simple workaround is to tweak the “mail.content_disposition_type” setting in Thunderbird. Click "Tools" and select "Options." Click "Advanced," "General," and then click "Config Editor." Change the value of “mail.content_disposition_type” to "1". Close and restart Thunderbird for the changes to take effect.
If you have difficulty editing your Thunderbird settings using the Config Editor tool, navigate to the “prefs.js” file in your Profile folder. This file stores changes made to the default settings. When you modify your email preferences, the settings are written back to disk and stored in the prefs.js file. Add this code to that file:
user_pref ("mail.content_disposition_type", 1);
Close and restart Thunderbird for the changes to take effect.
Warnings and Considerations
It's risky to modify your email configuration by directly editing the prefs.js file from the Profile folder. You could render Thunderbird and other Mozilla applications unusable if you make just one coding mistake. For this reason, it’s best to use the “Tools” menu to make changes as previously described.
The prefs.js file is used by all Mozilla programs installed on your computer, which may include Firefox, Mozilla Suite and SeaMonkey, as well as Thunderbird. Any changes made to the prefs.js file will also apply to those applications, if installed.
If you continue to encounter problems sending a particular email attachment to a specific recipient, contact the individual, as his email client might not be configured to accept inline attachments. This practice is common among users who prefer to eliminate the risk of being infected by viruses concealed in email attachments. If the problem remains unresolved, contact Mozilla for further support.