Multi-domain aliases not signing correctly

[barkerest]

Versions

• Windows Server Version: 2019 Standard 17763.4252
• Exchange Version: 15.2 (Build 986.37)
• Installed DKIM Exchange Version: 3.4.0

Description

When sending as an alias on a multi-domain setup, the signature is applied from the "home" domain instead of the alias domain.

Steps to Reproduce

1. Have multiple domains configured (eg - alpha.com, bravo.com).
2. Have a user with multiple aliases spread across the domains (eg - [email protected], [email protected]).
3. Have the user send a message using one of their aliases (this can be done via an SMTP session, an addon, etc).

Expected behavior:
The message should be signed using the keys for the sender domain.
If the user sent their message from [email protected], the message should be signed using the keys from bravo.com.

Actual behavior:
The message is signed using the keys from the user's primary email address.
If the user sent their message from [email protected], the message is being signed using the keys from alpha.com instead.

[Deckard99]

I can confirm this problem. The tansport agent seems to use RFC5321.MailFrom (envelope) as signing domain instead of RFC5322.From.
As nobody seems to care for this tool anymore in regards of development, I guess this will probably stay this way.