Can you let us know what ZD version are you using?
Can you resolve the zimbra.imap.mail.yahoo.com name?
nslookup zimbra.imap.mail.yahoo.com
or
host zimbra.imap.mail.yahoo.com
Have you tried connecting to the name via telnet?
telnet zimbra.imap.mail.yahoo.com 993
telnet <NAME> <PORT>