My mail client can't not sync more than few hundred emails from Zohomail

My mail client can't not sync more than few hundred emails from Zohomail

Hello,

My client is no longer able to connect after syncing the most recent few hundred emails.
For some reason, Zoho returns "Service Unavailable".

Thu, Dec 21, 2017 at 4:21 PM Authentication error. Server returned error "Service Unavailable"
Thu, Dec 21, 2017 at 4:17 PM 200 mails fetched.  2014 mails remaining.
Thu, Dec 21, 2017 at 4:15 PM 200 mails fetched.  2014 mails remaining.
Thu, Dec 21, 2017 at 4:13 PM 200 mails fetched.  2214 mails remaining.
Thu, Dec 21, 2017 at 4:10 PM 200 mails fetched.  2414 mails remaining.