I have domain domain.com in smartermail. Also have 2 emails one@domain.com and two@domain.com.
When I configure account in microsoft outlook there is fields one just email address and second for authentication where I wrote email address and pass.
if first address and second address different but both is exist in domain.com, smartermail allow send mails. if in first first I wrote one@domain.com but in second two@domain.com, I'll able send send email as "one", but authenticate as "two".
There is problem everybody can change first field to someone else address and send emails as him.