Just doing some googling, it may be that the SMTP server has changed it's configuration.
If your SMTP server requires username and password, you can try going from "myusername@domain.com" to "myusername" sometimes an SMTP server is set to require the domain, other-times not.
I would contact whoever controls your SMTP server and have them help you debug it.
I don't see any bug fixes in 9 and 10 for the Email sending, so probably not a kentico bug.