This could be simplified by use of rcube_utils::parse_host() which is what we already do in Syncroton and iRony.

Ha, I missed we use parse_host() in select_host(). So, that whole change in login() seems redundant.

On the other hand we use default_port everywhere, and we probably should not. I'm just wondering, why that second part of this change is not needed in Syncroton and iRony, but is in Chwala.

It's a bit of a mess indeed.
I think it's not needed because default_port will be ignored if the port is part of the host string.
I'm unsure how to clean this up without breaking anything, so I'll just stick to doing what we do elsewhere.