> I added a second MX record to my domain, pointing to a different server. The records point to 'mydomain.com' with priority 1 and to
> 'secondary.com' with priority 10. I was assuming that email would be delivered to 'secondary.com' only if 'mydomain.com' was not available.

> However, I am seeing some email delivered to 'secondary.com' even though mydomain.com was up and receiving email at the time. Some of
> this email comes from legitimate bulk mailers (e.g a newsletter from an association I belong to). The rest is spam. I've even seen one or
> two delivered to primary AND secondary.
>
> I'm curious whether the mailers sending to the secondary are broken, or whether I misunderstood the semantics of the priority in the MX
> record.

You understand the rules for a MX host, however many spammers automatically connect to the highest numbered MX host to bypass the anti-spam settings of the primary host.  In addition, even if your primary is up there may be something you're are doing to reject and then many host just go to the next higher host even when they get a 500 series fatal error message.

Personally I use the following MX settings where the highest host points back to the primary to try and bypass some of this.  ;-)

  tstephenson.com, MX, 10, mail.tstephenson.com
  tstephenson.com, MX, 20, mailrelay.bayarea.net
  tstephenson.com, MX, 30, mail.tstephenson.com
 

I added a second MX record to my domain, pointing to a different server. The records point to 'mydomain.com' with priority 1 and to 'secondary.com' with priority 10. I was assuming that email would be delivered to 'secondary.com' only if 'mydomain.com' was not available.

However, I am seeing some email delivered to 'secondary.com' even though mydomain.com was up and receiving email at the time. Some of this email comes from legitimate bulk mailers (e.g a newsletter from an association I belong to). The rest is spam. I've even seen one or two delivered to primary AND secondary.

I'm curious whether the mailers sending to the secondary are broken, or whether I misunderstood the semantics of the priority in the MX record.