You or your customer have a global block on emails sent from servers with the domain "mktomail.com." You would like to keep the global block but allow email from your own Marketo instance to pass through.
The return-path (or envelope_from) in the header of most Marketo emails includes the domain "mktomail.com." Some email servers may have a global block on this domain which prevents delivery of your Marketo emails. There are two options for bypassing this global block.
Contact your account manager to see about added the Branded Envelope_From feature to your instance. This will replace the "mktomail.com" domain with a domain of your choice.
Have the email admin add a Regex version of your return-path/envelope_from to the Allow List. Which Regex you use will depend on which datacenter houses your instance.
To determine the datacenter, look at the URL for your Marketo instance and see which letters come after the "https://app-". "SJ" is for San Jose, "AB" is Ashburn, "E" is London, "SN" is Sydney
The Regex you will use is as follows:
San Jose datacenter - "munchkin_id.(\d+.)*\email@example.com"
London datacenter - "munchkin_id.(\d+.)*\firstname.lastname@example.org"
Replace "munchkin_id" with the Munchkin ID for your Marketo instance. This operates as the unique identifier to allow only email from your Marketo through the block. Other email from the "mktomail.com" domain will still be blocked.