I try to explain.
So, we have a main Exchange server in our company with about 800 mailbox.
In our network, for security reason, Exchange can't send mail directly.
For sending mail outside our domain, Exchange must connect to iRedMail.
iRedMail can be considered as a gateway.
Before we used MDaemon as gateway but we decided to replace it with iRedMail.
When a user sent a mail with Outlook connected to Exchange, we could see this message a few second after in MDaemon.
Now, when a user sends a mail, the mail stays into Exchange and makes more 2 or 3 minutes to be seen in iRedMail.
My colleague sees that Exchange seems loose his connection to iRedMail.