The error message is from your email provider's server. You best bet is to talk to your email provider.

Hi christ1, first of all let me thank you for your suggestion. I will try to contact tin.it I wish to add another scenario I tested this morning: If I log-on on my tin.it page (webmail scenario) I have not problem to send mail with attached file. Do you think it is a problem between Thunderbird and SMTP server (settings)? Thank you in advance Felice

If I log-on on my tin.it page (webmail scenario) I have not problem to send mail with attached file.

Webmail is different server infrastructure than POP/IMAP and SMTP servers. Just because webmail works doesn't mean SMTP works too. There can still be a problem at the email provider's end.

Do you think it is a problem between Thunderbird and SMTP server (settings)?

It may not be a general problem with the SMTP server, but rather some kind of filtering or security measure on their end. It could even be something specific to your account. Only your email provider can tell why your messages are being refused.

Hi christ1 I will look into the matter further. Thank you very much for your support. Felice

Sorry christ1 I don't want to take advantage of your patience and I apologize for boring you again. I wish to share two more tests: - I sent an email with attached txt file. It works; - I sent the unsuccessful previous jpg attached file, but now it is a ZIPed file. It works; - unzipped file is 4278KB - zipped file is 4224KB not so different!! I'm not a computer expert but I give up trying to understand why it works by introducing this change. Thank you ik8wtd

This kind of fits into the picture.

1. There is no general Thunderbird problem with sending attachments.
2. tin.it presumably are selectively blocking certain file types when attached to an email.

The good news is, you do have a potential workaround for problematic file types - just zip them.

christ1..again..thank you so much!!