Emails are not sending out if their SAP user-id is being used as recipient, the email is not present in SOST,
when the direct email address is entered as the recipient address, it can reach the external mail system.
As of Basis Release 6.40, we can use transaction SO16 to check and maintain Forwarding/mail system group
addresses on the 'Mail system group' tab page.
If 'No Mail System Group' is set and no autoforwarding setup in SO12, emails won't be sent to external mail system.
1. In SO16 -> Tab "Mail sy.grp, Select radiobutton "Send to Home Addresses of User.
2. In transaction SU01, the user must has the email address maintained and have "Email" as the Comm.Method.