After much struggle with all clients known to me, I now use Thunderbird.
I might not keep using it for a longer time, so I kept the old
configuration and disabled it, so it can be reused at a later time.
Because Thunderbird doesn’t support nice declarative configuration, I
don’t see much sense in configuring the accounts declaratively, when the
settings for the accounts (which is much more) have to be configured
manually.