Recently I wanted to set up my student mail account on my PC, but I could not get a connection to the IMAP (outgoing) server. Reading emails via SMTP worked seamlessly, but no matter what connection details I used, thunderbird printed an error message.
The same issues occured when tried on my laptop and smartphone, so it seemed like a server problem, maybe external tools are not allowed (which would be kind of awkward). A friend of mine had no connection problem, so it had to be a problem on my network.
After some time of annoyed trial and error and thinking, I remembered something I came across some time ago.
The culprit could be the router, a Speedport Hybrid. There is a setting called E-mail abuse detection. On first sight, this seems like a good thing, after all it tries to do something against email spam. But the problem is the way this ‘detection’ works. It is simply a whitelist of ‘good’ and known IMAP server. So, any connection attempts to unknown IMAP server are simply silently blocked. No wonder I could not connect to my email account. After switching this ‘feature’ off, everything worked smoothly.
My reasoning for switching it completely off and not just whitelisting the needed IMAP server is, that it's just overkill. Of course, it might block a lot of rogue email spam, but it also limits the decentralised web. Everyone with his own or not well known email server will have issues behind this router. Probably without even knowing, that the router is the cause of trouble.