DMARC status of onetel.com

[rm454]

Unable to send emails to Corpus Christi College Oxford Development Office. 

Recipient's email provider returns rejection message:

Generating server: CWLP265MB6036.GBRP265.PROD.OUTLOOK.COM

corpdev@nexus.ox.ac.uk
Remote server returned '550 5.7.509 Access denied, sending domain onetel.com does not pass DMARC verification and has a DMARC policy of reject.'

[rm454]

Hello Ady

I am using port 587 of smtp.talktalk.net from the email client program MailClient on a desktop computer running Windows 7 Professional. 

Some weeks ago  smtp.talktalk.net started rejecting my emails to all recipients.  After some experimentation I was apparently able to remedy this by changing my Security policy setting from 'Force usage of SSL/TLS ' to 'Don't use secure connection'.  This has so far worked satisfactorily for all recipients except the Corpus Christi College Development Office.  I informed them of this problem using my Hotmail account and they replied as follows: 

"I’ve spoken to our IT team and I’m afraid they can’t see an issue on our end. From what they can see, it looks like onetel have enabled DMARC, so they’re not sure why the error message you received said that it did not pass DMARC verification. 

They have suggested for you to get in touch with onetel to see if they can shed any light on this." 

[Ady-TalkTalk]

Hi rm454, are you using mail forwarding or sending direct from the smtp.talktalk.net port 587? 

Ady