I'd like to add another possibility of causing this Error (ME740), although this thread is already answered. I had the same issue, an I checked every hint given in this thread.
The reason i got this error was a setting in SCOT in the SMTP node: Check the Settings for the "Supported adress types" an then in "internet" (checkbox needs to be filled). Then check, if the Email or Email-Domain you used in SU01 is mentioned in the "address area " Box.
All sender adresses need to be mentioned in the "adress area". Otherwise the Mail will not go out.