You came here probably after seeing a mail of yours being rejected with a message like this:
Greylisted, see https://stwalkerster.co.uk/support/greylisted/ or wait 300 seconds
As part of our spam prevention measures, any email from an unknown sender is “greylisted”, or delayed for a short period. This relies on mail servers being standards-compliant and respecting the retry status code returned by the server. Normally, your mail server will queue the message and retry after a short period, after which the message will be accepted, but most spam senders will not wait and give up after the first attempt.
If you feel this is blocking legitimate mail from reaching it’s intended recipient, then please do get in contact.