What is the exact error message?

thus: The email system had a problem processing this message. It won't try to deliver this message again.

this is the response from my ISP: i have re-installed thunderbird; what my ISP suggests is a bit beyond me..

Hello:

I haven't seen this issue before, but here is the key to it: "Failed to process message due to a permanent exception with message. The message content has become corrupted. MimeException: MIME content error: Wrong magic number in Apple MIME attachment."

It appears that your attachment is of Apple product origin and are being corrupted when they are sent by your Thunderbird client to another mail server.

From my investigating, I think the problem is with the default encoding process for attachments in your Thunderbird client.

If you have the ability to change your attachment encoding in Thunderbird, you may want to try that. We don't provide specific support for Thunderbird, but here is a support page for it:

https://support.mozilla.org/en-US/kb/configuration-options-character-encoding

If that doesn't fix the problem, we could have one of our mail admins take a look in the morning as well.

Thank you,

Scott Skeels Tierpoint Support

Do you have an anti virus program installed that is scanning outgoing mail? Most of them are fairly poor at what they do and often mail issues trace back to them corrupting contents, or interrupting connections with the SMTP server.

The fact the error is talking about apple mime is an issue in itself that leaves me asking do you have a mail signature? Was it generated in Thunderbird's composer as recommended, or in another package that generates weird proprietary MIME encoding.

Are you digitally signing or encrypting your outgoing mail? another failure point as some networks will not allow encrypted mail as the Boss can not read it.