Anti-virus software often causes these kind of problems.

These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

This is nor an accceptable answer. I will not stop my emails from being virus checked - I will not open a door to virus/trojans etc into my system. Do you know that most of the Virus comes from emails?

If it was the Kaspersky then why does it work sometimes and not others.?

I will create other email address to see if perhaps it is the provider taking too long to answer.

Further information to let you judge if AV scanning of email is worthwhile:

http://kb.mozillazine.org/Email_scanning_-_pros_and_cons

http://kb.mozillazine.org/Antivirus_software

The timeout could happen even with no attachments if Kaspersky is scanning the secure connection, another 'feature' that should be disabled.

I think scanning EMails is vitaliy important because emails are a primary source of virus+spam etc. So not checking them is not on - and is,in fact, asking for trouble.

Please note that sometimes the sending of an email woks well but occassionally it seems to stall and not end. This does not seem to be a AV problem since the message does get sent - but not copied into the Dent folder.

I am woundering if transfering of the email being sent to my provider (telekom) is playing a role here. Perhaps a large ping time?

Note that disabling the scanning of mail folders does not mean that the real-time background scan is disabled, in the event an infected attachment is opened. Also, TB doesn't run scripts or allow remote content, by default.

There could be an issue with your provider, but the first thing to try is to disable, temporarily at least, AV scanning of mail folders and secure connections, and see if performance improves. If it doesn't, you can always go back to your previous setup.