How to check if DKIM is working on a domain?
Verifying DKIM setup on a Plesk server
Verify that DKIM is enabled in server-wide setting at Tool & Settings > Mail Server Settings.
Verify that DKIM spam protection system is enabled for a domain at Domains > Mail Settings.
Verify that DNS TXT records in domain’s DNS zone exist at Domains > DNS Settings or on external DNS server:
Using the “nslookup” utility (works both on Linux and Windows in a command prompt), verify that the records from step 3 have been propagated to the Internet. Replace example.com with your domain name:
C:> nslookup -q=TXT default._domainkey.example.com 126.96.36.199
default._domainkey.example.com text = “v=DKIM1; p=<DOMAIN’S_DKIM_KEY>“
C:> nslookup -q=TXT _domainkey.example.com 188.8.131.52
_domainkey.example.com text = “o=-“
184.108.40.206 here is a Google Public DNS.
When sending an email to external mail services (Gmail, Outlook, etc)
Send an email from an email account on your domain to external mail service.
On external mail service, open email header and verify that there is a record
If all the steps are met, DKIM is working fine on the domain.