I have a gmail and ISP email accounts. Gmail account continues to work fine.

Last evening ISP account in Zimbra quit accepting messages, after accepting some earlier in day. I also learned this evening that one can not send a message either using ISP account.

ISP email accessed through ISP is being received, so I was able to retrieve messages that Zimbra quit accepting.

I have not made any changes to email settings through ISP nor through Zimbra.

Does anyone have any suggestions as to why ISP email account quit? Thanks