Exchange Server bounce back when external Client's email the business.

Hi, 

One of our clients are having a small issue and I have been unable to pin this down. The businesses Broadband line was changed over to Fibre a week ago and since then, some of their clients are having an issue emailing our client.

When client's email the business they are receiving the below error. Not all clients are having an issue sending & there is no logical pattern to it.

'The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk'

My colleague switched the line over and updated all the relevant records where the domain is hosted. One thing we have noticed is that everyone in the business is receiving a stupid amount of spam emails now.

Any suggestions or help is appreciated.

Thanks,

Alex