IF you have an issue about your DNS not allowing you to add a DKim key because the number of characters is too long, we found an alternative solution for this situation.
What needs to be done is to create an account in a FREE DNS service that allows long character keys, they are multiple services that offers you FREE DNS zone files one of them is https://www.cloudns.net/
You sign in for a FREE account
Once you have created your FREE account, you will need to add a FREE DNS zone
Then Select FREE Zone, this is for this service, some other sites might have different options
The next step is to create a domain and to be clear this is just a name not an actual domain name in my case I used
testdkim for the domain
The New DNS zone files gets created
Add new TXT record
Here you need to pay attention this step is particularly important.
For the Host name you need to Enter the name of the Selector from Proofpoint Dkiim
Value of the Dkim key
Once you have the above then go back to the TXT record and enter the information and it must look like the below screenshot
We need go to your current DNS zone files and create a Cname to redirect it to the cloudns that has your Dkim key
Select ADD new Record
Select Cname
Host will be the selector from Proofpoint Dkim key selector-16154XXXX7._domainkey
Points to is the TXT host name in Cloudns including the subdomain name previously created
Save the File and wait for your DNS to replicate, cloudns is fast to publish the txt record.
You will need to verify the Dkim key in Proofpoint
You will get a message in Green Verified, also from a public website you can check the dkim key
https://protodave.com/tools/dkim-key-checker/
Enter the selector and domain name