[CRITICAL] cannot SEND e-mail

[CRITICAL] cannot SEND e-mail

I was very unhappy to discover that I cannot send e-mails. Neither by webmail nor directly through SMTP. Messages appear in the webmail's "sent" folder, but none of them really hit the target.

In other words: outgoing mail just stopped working today. This is a CATASTROPHE.
Anyone else experiencing the same issue? Zoho team - huh?