Authenticate outgoing server email?

Hi,

For a different server and location I support I use a comcast username and password inthe form of

username-   xxxxxxxx (oneword, not an email)

pw-

I now have a new server at a new location and am trying to get server health reports to be sent outgoing to smtp.comcast.net.  I am using TLS port 587 and the username for the account in the form-

username- name@domain.com

pw-

This fails to authenticate, however, when I use the other clients comcast username and pw at this location, it sends.

When I asked Comcast support about getting a username for outgoing authentication they said to use the email login for the account.  Which doesn't work.

What is missing here?  There is no on-premise email server and there are several devices I want to send outgoing email from.

Thanks for any help,