Postmark SPF record

Publish it as a TXT record on the domain itself — the root, written as @ in most DNS panels. Not on a subdomain, and not on a _spf name.

One record only. A domain must have exactly one TXT record starting with v=spf1; a second is a permanent error, and receivers then treat the domain as having no policy at all. If you already publish SPF, add include:spf.mtasv.net to the record you have rather than publishing another.

Sending through more than one service? Each include costs a DNS lookup and the specification allows ten in total. Add every sender to the same record, and keep an eye on that count.

The ~all at the end is a soft fail: mail from anywhere else is treated as suspicious rather than refused. Once you are confident the list is complete, tighten it to -all.

After publishing, confirm it is live with a DNS lookup — a TXT record can take as long as its TTL to replace the old value.