Greetings, we have a single mail domain with two mail hosting providers and two ISPs. We set it up this way so that if an ISP or Mail provider has issues we can fail over to the other. We use a sonicwall for this function.
Users have OE6 clients with two accounts configured with the same email address but different smtp & pop settings.
Occasionally we send mail out and it is not received but we get no failure notice. If the user switches to the other account and sends to the same recipient they may receive it fine.
At first we thought we were going into a spam bucket on the recipients end but other users here can send to the same recipients fine while others experience the lost mail issue.
We additionally discovered that mail sent through the web based user accounts that fail from the clients go through fine.