A bug in our migration code, which was done as part of the infrastructure upgrade resulted in the IMAP issues you are facing.

Since it corrupted the IMAP UID, it resulted in complete resync of data for impacted IMAP users. 

Though we identified the issue and corrected it before the impact affected all IMAP users, we could not do much to help the first batch of users who were affected. 

This resulted in high volume of resync requests which stressed our resources and resulted in slow response and impacted mail reception too for some users. The resync and restoration time at the IMAP client side is dependent on the bandwidth of the users as well as on the way resync is handled by the respective IMAP client. This in combination with our slow responding servers, resulted in further impact for users even after we had resolved the issue at our end. 

Further, a natural calamity ( https://www.zoho.com/service-updates/blog/zoho-customer-support-slow-due-to-chennai-flood-disaster.html) made matters worse as we were working on a very lean strength the past few days, both in development as well as in support. 

But, again we take full responsibility of all this.