Version:           1.9.1 (using KDE 3.5.0, compiled sources)
Compiler:          gcc version 3.3.4 (pre 3.3.5 20040809)
OS:                Linux (i686) release 2.6.8-24-smp

When deleting an email account that is explicitely activated for mail processing in a filter it is not possible to restart kmail again. The last message in a console is: kmail: pPopFilter set

Removing the account id from the mail filter inside the kmailrc fixed the problem

The filter definition causing the problem was something like:
[Filter #xy]
Applicability=2
...
accounts-set=...,...,<deleted account id>,...,...
...

The <deleted account id> should be deleted together with the account.

As a side note the icons of the filters that were added to the toolbar disappeared after deleting the email-account from the GUI but reappeared again after 'fixing' kmailrc.

Thanks,
Jens