I can receive emails, but I have not been able to send any emails today. I keep receiving the following message each time I try:

Mail Delivery Status Notification (Delay)

ResponseCode 421, Host not reachable

I don't think this is due to the recipient's host, because these emails are being sent to clients of mine to whom I have previously sent many emails to the same email address without issue. Further, it would seem too coincidental, given that I am receiving this Delivery Error in respect of emails sent to a range of different businesses including: ...@stockdaleleggo.com.au, ... @clplawyers.com.au, ... @vicbar.com.au, ... @accesstraininglogistics.com.au,

I am trying to run a business which is heavily dependent on being able to send emails. I hope that someone can help me asap!

Thank you

Matthew