I'm having some mail issues which I'm trying to resolve. When I connect to my mail server using telnet should I supply my username like this:
or doesn't it matter?
It depends on your configuration. If you are delivering mail to people who have shell accounts, you must specify the username, and not the email address (the domain is implied, and dovecot won't know what to do with it).
If you are delivering mail to virtual mailbox users, and the virtual mailboxes are mapped using the full email address, you need to give the email address there. It all depends on how you've mapped it.
I have a setup where some users have their unix username as their login (as they have shell accounts) and others, on some hosted domains, use their email address. These configuration directives make it recognize unix usernames:
These make it pull the email address and password out of an SQL database, and then find the mailbox for the username in the filesystem:
It's quite flexible.