You may want to open a ticket with your mail provider to see if they still have our IP range (5.2.17.178-190) white-listed. This would normally be the reason why you'd suddenly encounter a timeout issue when there has been no change to the underlying credentials.
EDIT:
The SPF record won't have any affect on the SMTP time-out, it will just add a record to your DNS that any recipient can lookup to tell them it's OK for your mail to arrive from our servers.
I did a quick check on your mail server and it's not responding at the moment:
http://mxtoolbox.com/SuperTool.aspx?action=smtp%3amail.zoho.com&run=toolpage