If you use IMAP protocol this means you can only sent emails using your primary email adress, as specified in the staff catalogue (personalsok.su.se). These settings are going to be compulsory from the autumn, but you are encouraged to start using them as soon as possible.

Imap server/Incoming
Imap server/Incoming server: ebox.su.se
Port: 993 SSL/TLS
Authentication method: Password Username: University account details

Outgoing e-mail, encrypted smtp
smtp/outgoing server: ebox.su.se
Port: 587 STARTTLS
Authentication method: Password

See also: https://serviceportalen.su.se/en-us/article/606706