Record when you requested or triggered the email and when it arrived, with your time zone. Check spam and filtered folders once, then compare other messages from the same service if you have them.
Avoid posting your address, reset links, or codes. Do not request many copies in quick succession, especially for security emails where a newer request may invalidate older links. If the message never arrives, report that separately from a measurable delay.
What type of message are you waiting for?
Akousa