Can't share google calendar due to domain's DMARC policy

[jj_56]

Message blocked

The response was:

550 5.7.26 Unauthenticated email from lineone.net is not accepted due to domain's DMARC policy. Please contact the administrator of lineone.net domain if this was a legitimate mail. Please visit https://support.google.com/mail/answer/2451690 to learn about the DMARC initiative. 

The calendar is about 10 years old and only recently had this problem with sharing.

[Gondola]

I see you haven't been signed in to Community for over a day and haven't responded to my reply in Community. Do we assume you no longer need help and can close this topic?

[Gondola]

Gmail will check incoming mail for authentication and will reject all mail that fails to meet their authentication requirements.

TalkTalk Mail is capable of full authentication using SPF, DKIM and a DMARC policy. So the bounceback error from Gmail indicates that your outgoing mail server settings are not correct.

1. Use TalkTalk Mail webmail to prove your mailbox is working correctly. 
2. Then tell me what your email client software is and what the Operating system of the device is.
3. Then take screenshots of your email client outgoing server settings, image edit to obscure the first part of your email address and upload here using the camera icon above this reply area.

I'll then be able to assess what you need to do to send fully authenticated mail from TalkTalk and your lineone mailbox.