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Knowledge base Tertiary-mx.co.uk

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.

Spam Protection

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 blacklist
  • 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.


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.

The standard email service charge is applied, £1.20 pcm