1. server side:
log to your plesk cp, go to Server>mail, under Relaying choose:
authorization is required
and tick SMTP and POP3
leave the lock time at 20 Min
and then press on SET.
server side part is done.
2. outlook side ( as outlook XP):
click on tools > emails accounts
choose ‘view or change existing email accounts’ and click on next
choose your existing mail server that is having the problem and click on change
click on more settings
choose Outgoing Server tag