There is no way to change the DKIM settings per alias.
When you send from a DOMAIN alias, SmarterMail will sign the header indicating the domain you are sending from. This should match the domain alias. You should see your headers looks similar to mine below:
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=domainalias.com;
So when this is received by a mail server, the lookup will go against domainalias.com, you will need to ensure that this domain name has TXT records for DKIM setup identically to the primary domain. The test should then pass.
I just tested this against Google using this configuration and it was successful, below are my headers:
Authentication-Results: mx.google.com;
dkim=pass header.i=@domainalias.com;
spf=softfail (google.com: domain of transitioning admin@domainalias.com does not designate xxx.xxx.xxx.xxx as permitted sender) smtp.mailfrom=admin@domainalias.com
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=domainalias.com; s=05312016;
h=x-originating-ip:content-type:mime-version:message-id:reply-to
:date:subject:to:from;
bh=mh62rHCe6g4qZz9UQcXu1iQun0kiCltW5byNIzRe0wI=;
b=KAqC+klpp/7SPYhqH/kz/ZQfwgtTmnUBPMAwwuJFl4oKjIZ3jvhw7evizaZT9wCHR
QouQ1VReCzYPPslFzW2/6usS4LERAwZDy1Rx3lzxMMQ/XPQUR5AcNScQ9WGqmFSQx
7XT3qJNxcZkSLWPUaSOQ9ogIQFIrPwKUNRJdEJdg1Cn4cjlmUY7Z3oqOiLbdZCoxa
XRELXXgr/YwFawI3ylWfw93cndPFXGBMQKbuBJZmIDVmHSJY+OCRahd0JTO1PJ7PS
S0j9IPSLdbXJOx2xudgDvzCoxiHrv53weiupkCEbAHYNed1wxKBPqC9WHY+b19rXb
wnuPUxkq0IUajQOTQ==
Article ID: 2258, Created: June 20, 2023 at 8:47 PM, Modified: June 20, 2023 at 8:47 PM