Internittent email failure

[#1]

Emails to outlook and hotmail e-addresses have often been returned to me with the message "... could not be delivered due to a permanent error ...". Sometimes the problem disappears after a few days/sometimes it doesn't. Now sending emails to talktalk is going the same way. What is the issue? Can Plusnet fix it? Does the Greenby migration sort it? Thanks all.   Peter

[#2]

Usually an email delivery rejection message has a file attached Details.txt
with a more in depth explanation of what caused the bounce back.