I'm trying to setup a mail server and then on mxtoolbox.com I was told:
Your domain's SPF record includes a sender mechanism type of PTR. The use of this mechanism is heavily discouraged per RFC4408 as it is slow and unreliable. Per email delivery best practices, it is advisable to avoid including PTR type mechanisms in your SPF record.
Which is fine. But then what should I do? It just tells me that I shouldn't do this, but then doesn't say what I should do instead.