I ran into this too. It seems after going from version 14 to 16 the DKIM broke. It seems in this version SmarterMail creates a new DNS entry. I have to update all my DNS records for the accounts I had. Before I used: secure._domainKey.domainname.com, now it seems and probably a good thing generates a unique key name, now it is something like 123456789._domainKey.domainname.com. I would disable it if enabled, then enable it. I have only seen I can view the key and name one time, so I am making a copy. Update my DNS records. I use the 1024 key I have not figured out how 2048 in BIND. The records (notes this for BIND) I use and I pass Google's tests:
_domainkey.domainname.com IN TXT "o=~;"
123456789._domainKey.domainname.com IN TXT "k=rsa; key provided"
_adsp._domainkey.domainname.com IN TXT "dkim=all"
I also have a DMARC record
_dmarc.domainname.com IN TXT "v=DMARC1; p=reject; rua=mailto:firstname.lastname@example.org"
I also have an SPF record.
These all seem to give me three thumbs up on Gmail's security.