No incoming mail from Gmail

No incoming mail from Gmail

Hello. I've only been using zoho mail for a couple of days but have been really impressed with how easy it was to get up and running with my pop3 (Thunderbird) client.

I've been receiving emails just fine for the last few days and, after running my own mail  server for the last few years, was relieved to let someone else deal with spam filtering and all that sort of stuff!

However, this afternoon, I stopped receiving emails from any Gmail account.
I have a couple of Gmail accounts I use for work, and have asked friends with Gmail to send to me - I've not received one of their emails! Is there a specific problem with Gmail? The emails are not in my pop3 client, nor available via webmail. I've checked the spam folders and there's nothing there....

As far as I'm aware, the MX records etc. are set up correctly - after all, I've been receiving mail.
I've checked with my domain provider and my two MX records are for mx.zoho.com and mx2.zoho.com.
I'm stumped!