forwarded emails to @****.plus.com failing

[#1]

I can email colleagues on their abc@****.plus.com (real address hidden) email account without a problem. However if I try to forward a message to them from an external account, itsupport@warsashsc.org.uk the forwarding gets blocked and he never sees it. 

Any suggestions how to fix this? the message we get is:

Delivery has failed to these recipients or groups:

abc@*****.plus.com
A problem occurred while delivering your message to this email address. Try sending your message again. If the problem continues, please contact your email admin.

The following organization rejected your message: mx.avasin.plus.net.

Diagnostic information for administrators:

Generating server: exch-smtp-out.livemail.co.uk

abc@****.plus.com
mx.avasin.plus.net
Remote Server returned '554 5.0.0 <mx.avasin.plus.net #5.0.0 smtp; 552 Spam Message Rejected>'

[#2]

Exactly what fo you mean by “forwarding” in this context?

Do you mean …

sending on an email you have received to the @*****.plus.com address

Or

Sending to another email address (user’s own domain hosted elsewhere) which forwards to @*****.plus.com



These can be referred to as forwarding but are very different.

Are there any attachments involved?

Are you the sender?

Have you tried phoning support? The call desk is now the point of contact for SPAM issues.

[#3]

Thanks for the advice.  I will phone support.

I was trying to Forward an email sent to my hotmail account from a third party to a plusnet email account.

Having explored this further using my plusnet email account which I rarely use. The forwarding message is blocked by the Spam filter.