MWEB blocks our email sending

We have been with MWeb for years and had good service. BUT, last week we were unable to send emails from the business email address. Message from System Administrator : Server Error: 550 rejected blocked sender. We logged a technical error on the website. We called, which is a long and laborious process. We emailed from a gmail account. Apparently they needed an ID number (after all these years!?) which we supp****. And apparently the email address had been hacked...someone else is using it!!???

But, a week has gone by and no change to the email ability!!? What now??? Not happy!!