If you send email from your own domain, some of it stopped arriving on Nov 5.
Outlook, Hotmail, Live and MSN started quietly rejecting mail from domains with no DMARC record. Small senders too. Microsoft never announced it.
Only some messages bounce, so you'd never notice. Invoices, replies and confirmations just never show up.
Check yours in 5 seconds:
dig +short TXT _dmarc.yourdomain.com
No output means you're affected.
The fix is one DNS record:
Name: _dmarc
Value: v=DMARC1; p=none
It changes nothing about how Gmail or anyone else treats your mail.