I tried to send an attachment with an exe extenison, and avast came up with a warning. I canceled it and ran a boot time scan as well as a through scan on the folder that contained the file I was trying to send. Both scan results came up clean, so once again I tried to send the email to myself this time and the same warning came up. By any chance does avast automatically mark all email attachments that have an exe extension as a possible virus?
Most probably yes (avast could check email attachments that have an exe extension);
But I’m not sure this is automatic… you should disable smtp scanning (outbound mail) or configure your Heuristic tab of settings.
If you could, add a screenshot of the avast error and we will be able to help you easier… 
All you need to do is put files like that in .zip folders.