Why mail from a new correspondent sometimes takes a few minutes.
WHAT IT IS
A receiving server temporarily rejects mail from a sender it has not seen before, with a 4xx code. Legitimate servers retry a few minutes later and the message is accepted. Much spam software never retries, so the message never arrives.
WHAT YOU SEE
The first message from a new contact arrives several minutes late. Subsequent messages arrive immediately, because the sender is now recognised.
IS IT A PROBLEM
Usually not. The delay affects first contact only and it filters a meaningful amount of spam.
It is a problem when you need immediate delivery: password reset codes, order confirmations, two-factor codes from a new service.
WHERE IT APPLIES
It may be applied by the receiving server, which for incoming mail is ours and for outgoing mail is theirs. You control only your side.
IF DELAYS ARE CAUSING REAL DIFFICULTY
Open a ticket describing the specific sender and the delay you observe. Include the headers of a delayed message, which show exactly where the time was spent.
