Every now and again either my incoming or outgoing mail doesn't respond so rather than have to go through the nightmare that is trying to get TT to resolve the problem I use Gmail but using my TT email address as the sent from, this is something I've done for numerous years.
This morning once again I couldn't send emails so I copied the content to my gmail and used my TT email address as the sender but they're now being rejected as blocked with the following.
Your message to xxxxxxxxxhas been blocked. See technical details below for more information.
The response was:
550 5.7.26 Unauthenticated email fromtalktalk.netis not accepted due to domain's DMARC policy. Please contact the administrator oftalktalk.netdomain if this was a legitimate mail.
The failure to send error message is from gMail's server. It's normal practice not to allow the sending of mail from a different domain other than gMail unless the gMail server has been set to authenticate the sending of your mail messages.
Usually gMail will use the error message to reject incoming mail to gMail addresses when the sending domain does not have a DMARC policy of how to handle error notifications. GMail explanation here.