For years (per google searches) it seems there are issues with creating exchange accounts on Mac OS because Mac will strip the domain off of the username when configuring the account. In the case of SM you cannot login without a fully qualified email address which I believe is incorrect behavior and I'm not even sure if there is some RFC out there that this may violate.
Take for example user @ domain.com. If "user" is logging into mail.domain.com that should identify the domain to lookup for the user account. In addition, when I create my user account the actual "username" which should be available as a login does NOT contain an email address. So in the example above for "user" the username is just "user" and no domain name.
If this behavior could be corrected and allowed to login to web mail or an exchange account or any email account by username (without domain name) then this would solve a lot of problems "out there" with email clients namely EWS on Mac OS. Maybe there needs to be some admin setting to restrict usernames within the server domain name accessed to allow this? I know such as my setup I have SSL configured for mail.mydomain.com so I can allow other domain users to use this mail server for secure access instead of them dealing with SSL certs BUT with certs being so cheap these days I would say that's a bad practice and users should be restricted to the domain "container" in which they are assigned.
I look forward to SmarterTools comment on this as quite frankly with this situation it is nearly impossible to setup EWS on a Mac. My workaround is to put in a bogus email such as firstname.lastname@example.org which then triggers a dialog allowing me to manually setup the account which I think is another very bad situation and not many will understand how to do it. I hope in SM 16 this can be corrected to resolve this long standing issue.