Specialist internet services · since 2001
The SMTP gateway puts our relay, anti-virus and spam filtering in front of a mail server you run yourself. Clean mail is handed to your server on port 25; if your server is unreachable, we hold everything and keep trying.
Every message addressed to your domain passes through the same chain before it reaches your server. Each stage can reject or quarantine, so most of what would have hit your server never arrives.
Prefer it as a diagram? The platform schematic shows the same chain — MX, anti-virus and anti-spam clusters, the storage facility, and the delivery paths out to IMAP, POP3, SMTP and webmail. Platform schematic (PDF, 163 KB)
If we can't connect to your server — whatever the reason — nothing is bounced back to the sender and nothing is discarded. The queue simply builds on our side until you're back.
That webmail window is the part people forget. If your office connection is down for the afternoon, someone can still read the urgent messages from a phone — and when your server comes back, the same mail is delivered to it as normal. You don't have to choose between reading it now and having it properly delivered later.
If there's a firewall in front of your mail server, it has to accept connections on port 25 from our delivery ranges. Blocking these is the most common reason a gateway that worked yesterday stops working today — usually after someone tightens a rule.
The first is our own range. The second belongs to a partner network that also delivers on our behalf — allow both.
Allow both, for TCP port 25 inbound. If you're restricting by single address rather than by range, you'll need to revisit it whenever we add capacity — the ranges are the stable thing to allow, not individual servers.
Enter the IP address we deliver to and we'll try to open a connection to port 25, the same way our delivery servers do. A failure here means the problem is between us and your server, not with your mail.
If mail is queuing and you can't see why, we can look at our side of the connection while you look at yours. That's usually a five-minute call rather than an afternoon.