Go to the My Domains section in your admin (reseller) account.
Add your domain name (without HTTP or HTTPS or www) and save it.
The system will generate two DNS records (TXT records) as follows.
(Replace the example.com with your domain name)
Add these DNS records in your domains DNS (from where you purchased the domains such as Godaddy or any similar domain provider).
It will take some time to propagate the DNS records, so wait for that time (propagate time may vary from to provider)
You can verify externally as well if your DNS records are successfully added in your domain's DNS or not.
dnswatch.info/dns/dnslookup?la=en&host=mailer._domainkey.example.com&type=TXT&submit=Resolve
dnswatch.info/dns/dnslookup?la=en&host=example.com&type=TXT&submit=Resolve
(Replace the example.com with your domain name)
1. Verify the DKIM record.
2. Verify the SPF record.
Once records will be added, click on the Verify DNS Records button to verify your domain.
You will get the successful verification message it will addedd successful or the error message if it is not.
Or
Make sure that your domains will be verified subbfully before you will use them in your mailing.
Kindly Note: You can use only those domain which belongs to you or your customer. (for which you have the access of their DNS)