Emails between us and a specific counterparty are delayed. To give you some idea, we send 10 messages and over 40% arrive with a 20 (or greater) minute delay. We even had two messages arrive in 50 minutes!
I'm measuring the delay based on the header fields for the message. And I am only taking the delivery times between their and our perimeter SMTP servers.
I've also sent test messages via a 3rd party web mail account and can confirm that this isn't a slow internet link on our side, The test messages delivery times are sub-minute.
I don't think we're on RBLs as I would've expected messages to never reach us. I've been unable to do a traceroute from our perimeter SMTP server to theirs because of our firewall. If this issue does escalate we may have to put a device outside of the firewall to do traceroutes.
I also checked who their ISP was and I can confirm theirs is UK based (as is ours).
I'm at a loss to know how I can continue to determine the cause of these slow deliveries. I'd hate to go back to the counterparty's IT team and say it's a fault on your end, this seems like a cop out without hard evidence.
Just to recap:
- I am calculating the delay by measuring the time it takes for their perimeter SMTP server to deliver the message to our perimeter SMTP server. This means anti-spam/virus checks or mail journaling has already taken place on the counteparty's end and have yet to occur on our side.
- There are no delays if I send emails to our systems via a 3rd Party Web Mail account.
- Our userbase aren't complaining of slow Internet access or slow email delivery from other counterparties.
Thanks in advance!