I am after a bit of help/guidance with regard to an IMAP email issue I am having. The email is hosted by a ISP and we have around 25 mailboxes all using Microsoft Outlook 2016/19 clients with IMAP accounts (mailboxes are all smaller than 5Gb).
They have been working fine until a few weeks back when users started complaining that email wasn't arriving in their mailbox in a timely manor. It appeared that when the emails did arrive they came in batches which was odd, further investigation revealed that if the user hit the manual send/receive button in Outlook the email would arrive. The mail accounts all have web mail access and the email is arriving in there as normal, no need to manual intervention. So I don't think it's a mail server issue but that said the ISP does do something funky with the mailboxes as we have some Exchange mailboxes mixed in with the normal Linux based mailboxes. We connect to these Exchange mailboxes with IMAP also and they exhibit the same issue.
I am working on the assumption that it's a LAN/WAN issue, this is because if the user is not connected to the office network i.e. at home email comes in as normal and there is no need for manual sync.
My knowledge of how IMAP operates isn't great, as I understand it the client connects to the server and when an email arrives at the users mailbox the server "pushes" it to the client, is that correct?
What I'd like to understand is how the "push" mechanism works and if there is any way I can monitor traffic etc to see if the servers are pushing or the clients just aren't listening.
Any thoughts on how I could approach troubleshooting the issue would be gratefully received.
Thanks
Rob
