Sometimes, after the SmarterMail password is changed, for Windows hosting we also need to update it in Plesk.

For Plesk 12.5, the password can be updated in:

Tools & Settings >> Server Components >> Mail Server (SmarterMail Server 15.x.xxx)

For old Plesk 9.5:

Settings >> Server Components >> Mail Server (SmarterMail Server 15.x.xxx)