Hi dojx,
Do you mean that your Android device can receive emails but can't send out after setting up Exchange email on your Android mobile? If so, please refer to the steps below to trouble shoot the issue.
1. Have a check whether the settings are correct at Android device.
For details, you can refer to this link: Set Up Exchange E-Mail on an Android Mobile Phone.
2. Here are some general tips based on your questions:
(1) Keep the domain blank/empty.
(2) Use m.outlook.com for the server or the exact server name found in OWA>Options>SMTP/POP3/IMAP settings.
(3) Use your email as the username (same thing you login in to portal.microsoftonline.com)
Thanks,
Keats Ge