SmarterMail 4.0 incorporates a new feature called "greylisting" which is designed to significantly reduce spam.

1. user@domain.com sends you a message
2. Smartermail responds with '4.5.1, please try again later' (This is acceptable according to applicable SMTP standards)
3. The sending server gets that error and queues the mail according to it's retry schedule ... somehere between 15 and 60 minutes is average (an SMTP server is *required* to do this according to the SMTP standards, but spammers typically do not do this). Our servers are set for a minimum of 10 minutes before accepting and a maximum of 360 minutes (6 hours) before expiring
4. When the time passes and the sending server tries again, Smartermail recognizes the message from earlier and allows it to deliver.
5. Smartermail puts the address into a 'good sender' list that it keeps for a number of days (currently set to 180), so any message between now and then from that sender will NOT be delayed.

Because of greylisting, you may experience what appear to be delays in receiving mail. After the first mail, however, this should disappear. It is also possible that some mailiing lists, etc. may not appear at all if they do not retry (and, if they don't, they are not following the relevant IETF standards relating to SMTP).

You can disable greylisting by logging into the web mail interface (http://webmail.yourdomainname.com), going to Settings ... My Settings and checking the "Bypass Greylisting" checkbox under the Mailbox tab.