Connections are refused or system is too slow to connect. For example, if IMAP is not running and you try to connect to IMAP directly you will see something like this:
telnet 0 143 Trying 0.0.0.0... telnet: Unable to connect to remote host: Connection refused
If you try to connect with a client, you will get a message such as:
“Client is unable to connect to the server at the location you have specified. The server may be down or busy.”