It's best to ask your system admin for the correct format of your Exchange username:
Microsoft Exchange + ActiveSync servers can be picky about the way usernames are entered. Your email address might be "firstname.lastname@example.org", and your Microsoft domain might be "abc_domain", but your correct username might take any one of the following forms:
To make things even trickier, you may have successfully entered something else as your username into your iPhone for the iOS Mail app. Your iPhone asks for an email, a username and an optional domain. But then it combines these pieces of information in a variety of ways, guessing at the correct format your Exchange server wants.
Talkler Now Guesses Intelligently
With the release of Talkler 1.3.7, the app now does the same, and guesses intelligently at the correct format your Exchange server wants. When running Talkler 1.3.7 or later, it's best to let Talkler guess at the settings initially (after entering your email, username and optional domain).
Just accept the username that Talkler guesses — even if it looks a little different from what you entered or what you see in the Settings app > Mail... We try to display for you exactly what your mail service sees (Apple doesn't always display the actual username as seen by your service).
The most surefire way to uncover your correct username format, though, is to ask your system admin.