For email addresses that are appearing with a '1' (or greater), after the SAMAccountName, e.g.: firstname.lastname@example.org, do a search for jsmith to confirm if there is already a record using email@example.com.
The template for mail@ADSAccount can also be customized so that email addresses conform to your company's requirements. In the case of managed accounts, where the value is inherited from the Employee, the template for DefaultEmailAddress@Person can also be modified accordingly.
For more information please refer to the Configuration Guide: Editing Value Templates.