Incoming mail delivery delays

In the last several days, I've been experiencing random delays in incoming mail delivery - a message would occasionally arrive several hours or days after the sender's timestamp. While these were low-priority automated messages from various forums, I didn't really pay attention, but just a few minutes ago, around 21:15 local time, I received an important e-mail from a client timestamped 10:49. I looked at the headers, and the problem is clearly on Zoho's side. The headers say:

Date: Thu, 15 Jan 2015 09:49:11 +0000
...
Received: from smtp***.***.com (smtp***.***.com [xxx.xxx.xxx.xxx]) by mx.zohomail.com with SMTPS id 1421315355599962.1097002150647; Thu, 15 Jan 2015 01:49:15 -0800 (PST)

That is, it took the message 4 seconds to reach mx.zohomail.com, and more than 10 hours to get from mx.zohomail.com to my IMAP mailbox! What's going on?