Mail to groups is not delivered

Mail to groups is not delivered

Dear Zoho Mail,
We have mail groups, like noc@domain (that domain is in Zoho Business paid edition), that point to emails on other servers (for example, noc@domain points to noc@rt.domain).

Everything was good until today. Users of Zoho Mail can not send to these groups, it's not being delivered, no errors, no mail returns.
But users not from our domain still can easily send mail to these groups, it's delievered.

Please, help us, we can't work and can't rely on services that are unexpectedly  changed time to time (we issued similar problem some time ago)

Thank you.