Hi. There could be numerous answers. The recipient's email client or provider may be rejecting it, thinking it is spam. Are you putting text in the Subject and Body?

What is the file extension? Possibly the recipient's email client or email provider is blocking those types of files.

Can you send this recipient a JPG or PDF attachment?

Can the recipient send you JPG or PDF or even that Annex file?

Can you send this Annex file to another recipient on a different domain like a Gmail or Yahoo account? Can you send the attachment to yourself?

The answer is probably simple but you will need to give us a lot more details, examples, and the answers to my above questions and suggestions.

The extension is .docx

I can send and receive JPG and PDF

The recipient can't send me the same file (she works also with thunderbird).

New problem: I cannot print all the messages in Thunderbird, f.e. your e-message from Mozilla support will not print.

Perhaps i can reïnstall Thunderbird ? But my recipient has the same troubles.

Well, let's stay on topic (please open a new ticket for your print problem).

She cannot send .docx either...but I don't know if she is on the same network as you or same ISP as you or same email domain as you...etc.

Try sending/receiving from someone who is using a Gmail or Yahoo account...and that person not being on your local network. Also, what about you using your ISP's webmail instead of the Thunderbird thick client?

If you have a laptop, you could also try connecting to a different network like your local library or coffee shop and if that works, then you know it is a network problem if it works on other networks.

These kinds of problems, again, are easy to solve but there are so many questions and so many trial-and-error scenarios you have to think about.