Internet addresses have the technical form
username@computername
The right part of the address is the domain, the left is the local part. To enable addresses in, for example, the form
firstname.lastname@company.com
to correspond, you must specify the host systemname.company.com as the mail exchange for the address company.com in the Domain Name System (DNS). Here you must create an MX entry, because in some configurations an alias can lead to messages not being forwarded to an SAP Internet Mail Gateway, but rather forwarded locally.

The line
Another possibility may be more robust, but involves more administration than that of an MX entry. You must also create an alias for every R/3 user on an existing mail server. An example of the entry in the configuration file Aliases of the Unix mailing program:
firstname.lastname: firstname.lastname@systemname.company.com
For information on how users assign addresses in the R/3 System see
Internet addresses.