Yet another email issue

[#1]

I have received a text message from my mobile phone provider (Giffgaff), as below

Important: We're unable to send you emails such as password resets, as your email was rejected by your ISP (error: "3"). Please update your email "me@myaccount.plus.com" for membername "myphoneaccount". Infogiff.ly/email_errorerror

This is appears to be yet another instance of Plusnet mail servers rejecting valid emails from organisations I subscribe to or have accounts with. 

Why does this keep happening?

[#2]

Apologies to anyone at Plusnet who may have made any effort to look into this. I have since been in contact with Giffgaff, who have investigated and confirmed that the text I received was sent in error.

[#3]

Hi @maranello,

Thank you for letting us know that this message was received due to an error on your mobile provider's side. Please don't hesitate to get back in touch if there's anything else that you'd like to discuss.