Authorising who can send.
WHAT IT IS
A DNS record listing which servers may send mail for your domain.
WHAT IT PREVENTS
Someone else sending mail that appears to come from you.
HOW IT WORKS
A receiving server checks whether the sending server is listed.
If not, the mail is treated with suspicion or rejected.
WHAT IT MUST INCLUDE
Your mail server
Any service that sends on your behalf: a marketing platform, a form service, an application
WHAT PEOPLE GET WRONG
Adding a service and forgetting to update SPF Having several SPF records, which invalidates all of them
THAT SECOND ONE
Only one SPF record is permitted per domain.
Multiple records cause failure.
Combine them into one.
THE LOOKUP LIMIT
SPF has a limit on how many lookups it performs.
Too many included services exceeds it and causes failure.
WHERE TO SET IT
Your DNS, wherever it is managed.
WHAT TO CHECK
That you have exactly one record, and that it includes everything that sends for you.
