Moved bug report from https://roundup.kolab.org/issue4239:

GpgOL sometimes sends

SENDER -- <kleopatra-does-not-allow-empty-sender@example.net>

This is GpgOL's attempt to access the "default signer" functionality of the
UiServer protocol spec. However, that functionality has never been and will
never be implemented in Kleopatra, because it's impossible to do so, since
Kleopatra doesn't have access to the Identity information stored in OL.
Therefore, GpgOL _always_ needs to send this information to Kleopatra,
irregardless of whether the mail is sent via the default account or a secondary
one.