Within Teamwork, you can set a default user to assign to emails sent from addresses not registered on your site.
These emails will then be shown in your site as having been created by that default user.
When you use the option to create items via email, by default those emails are only accepted from the email addresses registered for users on the site.
Click your profile icon in the bottom left corner of your site, and select Settings from the pop-out menu.
In the settings area, go to the Email tab.
You will see a section here for Default User Posting via Email. Click the link to Choose Default User here to use.
You will need to select from the site administrators, and once you've set this you can see which person has been selected and see a link to change them.
For more information, see: Site Administrators