Gmail rejecting delivery of emails

I have started to receive email delivery failures to any email I send to people with a Gmail address. It appears that the server with our mail service has been identified as being used to spread spam, and so now emails from the server are being rejected.

How can this be resolved?

Below is the report I am receiving for multiple Gmail addresses:

----

This is the mail system at host mc.internetmailserver.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<[email protected]>: host alt1.gmail-smtp-in.l.google.com[74.125.126.27]
said: 421-4.7.0 This message does not have authentication information or
fails to pass 421-4.7.0 authentication checks. To best protect our users
from spam, the 421-4.7.0 message has been blocked. Please visit 421-4.7.0
https://support.google.com/mail/answer/81126#authentication for more 421
4.7.0 information. 78-v6si8427882ioc.252 - gsmtp (in reply to end of DATA
command)

 

Thank you, support ticket now opened

 

Any update on this? Having similar problem.

 

This issue was resolved by adding an SSL certificate to my domain.