[exim] Error 5.5.2 : Invalid data in message

Hi!

I have this problem with some mails sent from Thunderbird, when the
message has an attachment with a name longer than 50 characters. Any other
client (Outlook, ...) has not the same problem.

If I use another SMTP server (GMail, Yahoo, etc), I can send the same
messages without problems.

What can be the problem with my SMTP server?