I lost some mail because I had 2 computers with kmail, with the same filters. The 2nd is just a copy of the first.
Using Dovecot as an imap server:
But somehow that is why strange things happened. One email had only the header and no message text.
When testing, I could see the mail entering the inbox and then it disappeared.
When I removed the filters from the 2nd computer, the problem was solved.

Ok, now my question, what can I do to prevent this errors?
I found 1 solution but should kmail not have a better conversation with the email server ?
--please try later.. another program is busy with this email --

Is it a kmail problem or is it a dovecot problem or both?
What if the pc with kmail and the filters, is not active? Then all email will stay in the inbox.

KMail processes mail locally if with filters, so I suspect that something went awry if the two machines processed mail at the same time. I would suggest, given you are using Dovecot, to set up Sieve-based filtering, so that it's the server which filters the email and not your clients. KMail has a nice configuration panel in which you can set Sieve scripts like regular filters.

Thanks einar, interesting, I did let kmail export a sieve script and that is a big help to start with.
Maybe I try this some day
It would certainly be a good solution!