I cannot get KMail to work.
I can set up Geary, Thunderbird and Evolution, without any problem. I know my email settings by heart. But I just cannot get KMail to work.
I've tried on most distros.
So I'm thinking there must be a gotcha that I don't know.
It takes the account settings. It says it's Online, but it just sits there and doesn't get any messages at all.

Any ideas?

What kind of an account is it? Gmail? MS Outlook? other?
What protocol is it using? IMAP? POP3? other?
What account settings have you configured around retrieval of messages?

Also, please include details about which versions of Kmail (and Qt) that you are using.