Hi Francisco,
Thanks for your reply - very appreciated. I would like to come back to this issue.
The emails suddenly downloaded again, don't ask me why and how, but since 3 days I am having the same problem again.
I have checked the settings in Thunderbird, emails are not getting deleted so I think it has nothing to do with thunderbird. I am having three e-mail accounts setup in fengoffice and two download without any problems. I have had a look at the data tables and new mails for the third account don't show up. The settings for all three accounts are identical and I am not getting any error message.
Do you have any idea/suggestion how to get to the root of this problem?
Thank you very much,
Markus
Update: Tested POP3 and it collects e-mails without problems. Still IMAP not working though.