For SMTP server it would be helpful to have senders IP-address in the mail header (iNotes, Verse). If mails are beeing sent outside the company, the IP-address can be checked.
This would allow to block mails sent from untrusted IP-address ranges (legal requirement). The header field can be removed to keep privacy.
A workarround would be to have Domino logging the clients IP-address for each mail submitted.
e.g. Thunderbird is adding this information:
Received: from [127.0.0.1] ([220.127.116.11])
by masked.x.z (IBM Domino Release 10.0.1)
with ESMTP id 2019061816184108-3 ;
Tue, 18 Jun 2019 16:18:41 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101