There is a bug in the Zoho IMAP server causing all messages to be returned when searching for messages by flag (UNSEEN, FLAGGED, etc), despite specifying a SINCE.
This causes all messages to be synchronized/downloaded, causing a lot of unnecessary traffic for both the user (inconveniently hitting rate limits as well) and for your servers.
See attachment for a protocol log demonstrating this rather serious issue.
The log was captured using imap.zoho.eu
07-24 17:07:32.020 26182 27215 I System.out: BG25 SEARCH OR SINCE 23-Jul-2019 (UNSEEN) ALL
07-24 17:07:32.034 26182 27215 I System.out: * SEARCH 1
07-24 17:07:32.034 26182 27215 I System.out: BG25 OK Success
07-24 17:07:32.035 26182 27214 I System.out: BG26 IDLE
07-24 17:07:32.041 26182 27214 I System.out: + idling
07-24 17:07:32.041 26182 27215 I System.out: DONE
07-24 17:07:32.052 26182 27214 I System.out: BG26 OK IDLE terminated
From the records we see that your account ... is not being accessed via IMAP from any IMAP client. Kindly help us with the email address of the Zoho account in which you faced this issue.
We have checked with our development team and the bug you have reported will be fixed in our upcoming updates. We will keep you notified once we get this fixed. Please bear with us until then.