We have persistent problems accessing several IMAP email accounts (all hosted by Unlimitedwebhosting) - we presumed it was a hosting issue and have had several sessions with their support but everything seems to be correctly set up.
As an experiment I simply switch off our wifi and connected via a tethered mobile phone instead. Immediately the account functioned perfectly
When (failing) to connect to IMAP via our Plusnet broadband the error message we get is
"could not connect to this IMAP server. Check your network connection and that you have entered the correct information in the Account Preferences. Also verify that the server accepts connections on port 143, if not change the Server Settings tab of Account Preferences"
This is all when access IMAP email accounts from various computers (all Macs using Mail) in the house and the problem effects iPhones if they connect via wifi too, but not via 4G. The accounts are all accessible fine as webmail. All other online activity is fine. One machine also connects to an Exchange account (hosted elsewhere than our IMAP accounts) and that account is unaffected by this issue. All the Macs access the IMAP account fine when we're out of the house and connect other wifi connections or 4G
The problem seems to happen intermittently - we'll be unable to access properly for days at a time - no sent /recieve. But every now and again connection is made and a bunch of emails all appear at once, a few minutes later we're locked out again.
Sometimes taking one machine offline / connecting via 4G seems to allow the others to connect so we're wondering if the problem is multiple machines connecting - but it if is its a problem, thats either happening at our router (Plusnet sagemcom) or at the ISP even though the error message appears to blame the server