You can specify a default domain to enable documents to be sent externally from and to users or business objects to which no Internet address is assigned. When Internet messages are sent, the system checks whether the sender has an Internet address. If the sender does not have an Internet address
The default domain can be used, for example, when a work item agent sends a mail over the Internet to another person. The reply is then sent back to the work item and appended to it as an attachment. As a result, every other agent of this work item has access to this information.
If you use the SMTP plug-in, you have to create a default domain, even if all users have Internet addresses.
If, for example, you specify company.com, the sender address firstname.lastname@example.org is created for the user testuser, to whom no Internet address is assigned in address management, when an Internet message is sent.