Greylisting is basically a server side filter that detects and removes spam emails. If an email is flagged as spam it can cause delays, or a delivery failure. The Greylist filter helps stop a lot of the spam that gets to the mail server, so disabling it is not typically recommended unless it has to be done.
Disabling Greylisting on a specific email account:
1.) Login to Smartermail.
2.) Go to Settings > My Settings
3.) Check Bypass Greylisting
4.) Select Save
To turn Greylisting back on, simply follow the same steps, but uncheck Bypass Greylisting.