The EM SMTP servers are set up to block all .eml attachments. This is because they can very easily contain a virus or similar payload. Unfortunately, a lot of email applications forward emails as an attachment with the .eml extension (which was originally Outlook Express's own file extension but has become a pseudo-standard used by other applications).
When you try to send the forwarded email, you will normally receive a message similar to the following:
- An error occurred while sending mail. The mail server responded: Potentially executable content. If you meant to send this file then please package it up as a zip and try again.
and the email will not be sent. When all you're trying to forward is a plain text email, it can be especially frustrating that it has been deemed "potentially executable".
The solution will depend on the application you use. Thunderbird is becoming increasingly popular across the different platforms, and is very easily customised.
- Go to the Preferences dialog (Edit > Preferences in Linux, Tools > Preferences in Windows) then the Advanced icon and the General tab.
- Click the "Config Editor" button to bring up the about:config dialog
- Type "mail.forward_message_mode" in the filter
- Change the value from its default value of 0 (send as .eml attachment) to 2 (send in-line with attachments included)
The only downside to this setting is that it does not allow you to forward multiple emails as attachments to a single email.
Instructions for Thunderbird 1.x are likely to be very similar.