Default domain (SCOT - Settings - Default domain)

Dear SAP gurus,

I just wonder if we can enter any domain name in the Default domain (SCOT - Settings - Default Domain).

I knew that if a user does not have email address entered in SU01, when he sends email out from SAP, SAP would generate the e-mail address by concatinating his username and the default domain.

Example:

If username is sapuser and default domain entered in sapdomain.com, so the e-mail address would become sapuser@sapdomain.com.

Now, if sapuser sends email out from SAP to external, say, yahoouser@yahoo.com, yahoouser can receive the e-mail. However, yahoouser cannot reply sapuser's email.

Does this mean that we need public domain (internet domain) to get external user to reply back sapuser's e-mail?

Rgds,

QT