Patrick Schmid [MVP] wrote:

AFAIK, there is no such option and why should there be? I'd suggest
you complain to web.de and get an RFC compliant email address.

That would be my suggestion as well. Simply because Outlook Express and
Thunderbird are not standards-compliant doesn't mean that Outlook should
match them, and if the OP wants to guarantee that won't ever be an issue, he
should get a compliant address.
--
Brian Tillman