Zoho Mail IMAP slow in Outlook

Zoho Mail IMAP slow in Outlook

We have been having an intermittant issue over the last month or so where an email takes a long time to appear in Outlook (connected via IMAP) using Zoho Mail. Up to date emails can be seen in the webmail client, but often take an hour or more to appear in Outlook, even after clicking "Send/Receive" numerous times. I'd really appeciate if you could advise what is going on as we need to be replying to emails ASAP. This is the same for emails between staff and emails from clients.

This may be a bit of a long shot, but would the timezone set in the Zoho Mail settings for a user make a difference? i.e. if one user is set to GMT, and the other set to GMT+3, there is potentially a 3 hour gap where between sending/receiving. In essence, the GMT user sends an email at 9am, which the GMT+3 user does not receive until 12pm. The reason I ask is I have done some fiddling with these settings and it does appear to help the problem.

Secondly, what is the difference between imap.zoho.com and imappro.zoho.com? We have a domain associated with this account but have been using imap.zoho.com, where as it seems we should be using imappro.zoho.com - could this also be a cause of the above problem?

Thanks