I've just discovered that it could be related to some lacking permission. I've tried to attach one of the archived emails to a message and when I send it I get the following error (see attached image):

Unable to save message as draft. An error occurred while attaching Re: Fw: Direct upload on AGAMI website.eml. Check that you have permission to access the file.

The attached message has a weight, but the path could be wrong as I see a "nobody" in the path while I should expect a username (see attached image).

Thanks for supporting me.

If the Thunderbird profile is specified as a UNC path, then you will be able to access files stored within that directory tree.

To access files outside the profile, you must set an exclusion by adding network hosts to hidden preference setting for mail.allowed_unc_hosts. For example hostname1,hostname2

Thank you for your support Wayne Mery. Could you please explain me where and how I can add network host to hidden preference? Thank you!

https://support.mozilla.org/en-US/kb/config-editor

Solve your problem?

Hi, it it's been helpful. Sorry for my late feedback!