Messages sent to Gmail being bounced

[#1]

Hello - for the last few days, messages sent from my Plusnet (metronet.co.uk) address to a relative’s gmail address have been being blocked by google. A copy of the error message and diagnostic is attached. It’s a 550 5.7.1 error apparently. 

Can anyone help?

thanks so much!

[#2]

Thanks for this, I'll get it looked into asap.

Fyi, I've asked the mods to delete the attachment as it contains your username. 

[#3]

Thanks on both counts