I'm pulling my hair out trying to come to a solution.
We applied for a fibre upgrade in January, which consisted of getting 2 new cloud phones and upgraded internet.
An openreach engineer came and put another phone line in late January, he mentioned that it was unusual that they marked the job as "New" phoneline rather than "upgrade" phoneline, but he did as he was asked on the jobsheet. Around the same time, we received a BT business hub and hybrid connect (but we already had one of each). I don't know if it's procedure to send these out again but it seems like a waste as they're not at all an upgrade from what we had before.
He ended up plugging in all four routers, and mentioned to me to double check that we're not accidentally paying for two packages as it could be the case. About a week later, we started having email troubles. Please note that these are our OWN IMAP emails hosted by our website company with the same domain suffix. I'm aware of the issues with Microsoft and BT emails - we are not hosted by BT.
I got in touch with our hosting company and they did some checks - nothing wrong on their end. So I went back to BT, and they said they didn't know what the problem is. They recommended I whitelist the domain on our broadband section of our account - I did - but nothing was fixed. BT were convinced it was the hosting company. So I went back to them AGAIN, and they connected to my computer, tried all manner of different settings and ports, but I still can't access emails. At this point our hosting provider said it wasn't them. I tried on my mobile hotspot on the off chance, and I could access my emails and webmail absolutely no problem. At this point I know it's BT wifi that's the issue, not my hosting company.
I went back to BT a second time, he asked his supervisor, we were on the phone for around an hour and a half, and still nothing is resolved. He said he's never seen this issue before.
I want to come to a resolution because I currently have two hubs, two hybrid connects, one cloud phone and I'm sure I don't need all of these. I still have yet to receive my upgraded cloud phones as part of the package (it's been over a month) and I can't access my hosted emails via Outlook, webmail (by typing it in on browser as a URL) unless I'm OFF this WiFi. I'm currently using my mobile data to access emails. Is there some strange firewall settings that's messing with me? Do I have a dynamic IP address that's messing with me? I did have a temporary fix of turning the router off and on which seemed to fix the problem but only for an hour or two before it went off again.
tl;dr: My hosted domain emails are being blocked by BT in some way. My hosted website can be accessed no problem, but my webmail, outlook or any other way of accessing my IMAP emails can only be accessed by disconnecting from this WiFi. Hosting company have connected to my PC, tried multiple ports and settings and they said the BT Wifi is the problem. Never had this problem on my old router but currently both old and new router having the same problem. Emails work fine when connected to any other wifi such as mobile hotspot.
Fixes I've already tried: