The hardest common problem.
WHAT THE TWO DIRECTIONS ARE
Mail not arriving to the customer Mail the customer sends not arriving elsewhere
WHY THEY ARE DIFFERENT PROBLEMS
One is about your receiving; the other about the recipient's filtering.
WHAT TO CHECK FOR MAIL NOT ARRIVING
Whether the server accepted it Whether it was filtered Whether the mailbox is full Whether forwarding is configured
WHAT TO CHECK FOR MAIL NOT REACHING OTHERS
Whether it was accepted by the recipient's server What the log says about rejection Whether the domain's authentication records are correct Whether the address or server is on a blocklist
WHAT AUTHENTICATION RECORDS MATTER
Those declaring which servers may send, and those signing messages.
WHY THEY MATTER SO MUCH NOW
Major providers reject or filter mail without them.
WHAT TO CHECK BEFORE ANYTHING ELSE
Whether those records exist and are correct.
WHAT TO EXPLAIN TO CUSTOMERS
That delivery is the recipient's decision, not yours.
WHY THAT MATTERS
It sets a realistic expectation without sounding like an excuse.
WHAT TO DO ABOUT BLOCKLISTS
Establish which, and follow the removal process.
WHAT TO FIND FIRST
Why it was listed.
WHAT THAT USUALLY IS
A compromised account sending spam.
WHAT TO DO ABOUT THAT
Suspend, change credentials, and find how it was compromised.
