Within the originating domain, signing can performed by?

Azuki

Using DKIM, signing is performed by the ADMD (Administrative Management Domain). Within the originating domain, signing
can performed by:

(a) MUA
(b) MTA
(c) MSA
(d) MSA and MTA only
(e) MUA, MTA and MSA

I think it's (e). Can someone help to verify?

Thanks!

rscrt

This is part of the Sec+ objectives? Never heard of it