SSL connection times out

I set up an SSL connection to an IMAP mail server using the default ports. Everything works ok for a few minutes (sending and receiving). After a while (few minutes maybe) I get a message that the connection has timed out. I can't see anywhere in either the Zoho or server setup where there is any timeout option. And the server doesn't time out when I am accessing it through webmail.

Any suggestions on what to try?