There is one thing to check, although this would be done strictly for username conflicts: Does the user name/email address in the 'From' section of your email (1) match the User name in your SMTP Settings (2) and the email address in your Server Settings (3)? 1 - Hamburger (top right)/Account Settings - click on the name of the account through which you are sending the email. On the right is the return email address called 'Email Address' unless you have put another email address in 'Reply-to Address'. 2 - Hamburger (top right)/Account Settings/Outgoing Server (SMTP). Highlight the server through which you are sending the email and the User Name is displayed at the bottom. It can be changed using the 'Edit' button. 3 - Hamburger (top right)/Account Settings - click on the 'Server Settings' under the account through which you are sending the email. The email address is in the 'User Name' field.