Replace a domain setup link
The link is the only credential your domain has, and it does not expire on its own. If it has ended up somewhere it should not be, replace it here. The old one stops working the moment the new one is issued.
Replace a setup link
A setup link is the only credential your domain has, so if you have lost it or shared it by mistake, replace it. You prove the domain is yours by publishing a value we give you. We cannot accept the record already in your DNS as proof: anyone can read that one.
Why a second record
You already added a _tmailr-verify record to set the domain up. We cannot use that as proof here, because anyone in the world can look it up. So we give you a fresh value instead: publishing it is something only whoever controls the domain's DNS can do.
What changes
Only the link. Your MX records, your aliases and the mail already flowing are untouched, and nothing stops arriving. Remove the _tmailr-reissue record afterwards.