tertiary-mx.co.uk is our backup email server. It will accept email for a domain we handle email for and will queue messages and will attempt to deliver to the higher priority MX record.
Tertiary-mx.co.uk does do spam checking and will defer email that looks like it is spam in some way. It will deny (cause a permanent delivery failure) messages/connections under the following circumstances:
- Invalid HELO
- Recipient domain is not one we handle email for
- Message size is over 1GB.
- The server will 'greylist' hosts. It will always defer (cause a temporary delivery failure) messages/connections under the following circumstances:
- Sending host in a DNS blocklist
- Header syntax errors
- Recipient rejected by the primary MX (It does a call back to the primary MX if it is available)
- Senders email address can't be verified
- Contains malware (including spam/phish signatures) as picked up by the virus scanner
- If the SpamAssassin spam score is over 6
The reason for deferring is so the sender is forced to use a higher MX, where it can then make the decision whether to accept or deny the message. -This gives customers running their own servers more control over spam.
Messages will be attempted to be sent on to a higher priority (i.e. lower MX record) email server starting at every 15 minutes, and increasing over time to every 3 hours. It will carry on trying for 31 days before bouncing the message back to the sender.
To force tertiary-mx to attempt to relay stored messages, telnet in on port 25 and issue an etrn command in the form of
$ telnet tertiary-mx.co.uk 25 Trying 2001:8b0:0:30::51bb:1e1d... Connected to tertiary-mx.co.uk. Escape character is '^]'. 220-relentless.aa.net.uk ESMTP Thu, 08 Sep 2016 09:30:29 +0100 220-Mail accepted subject to terms on http://www.andrews-and-arnold.ltd.uk/inboundemailterms.html 220 You are 2001:8b0:0::1 etrn #example.com 250 OK quit 221 relentless.aa.net.uk closing connection
Note that tertiary-mx has various measures in place to discourage spam. You need to wait until after the "220 You are 2001:8b0:0::1" line is printed before you can issue the etrn, which may take a few seconds. Mismatching forward/reverse DNS etc. may upset it when you try to telnet in. If possible telnet in from your mail server itself, as that is more likely to look trustworthy.
If you would like to use tertiary-mx.co.uk as a backup mail server for your domain please contact sales who will add the domain.
If you manage your own DNS server, once added at our end, then you'll want to add an entry similar to:
example.com MX 50 tertiary-mx.co.uk.