Recently, I have been experiencing issues with sending emails from Tencent Enterprise Mail to Gmail, while sending to other domain emails such as Yahoo works fine. I have been troubleshooting for a while. I found the solution at https://blog.csdn.net/JasonWeng/article/details/135085539.
The main issue was the incorrect setting of TXT record values, changing them as above resolved the problem.
Host Record: _dmarc
Record Type: TXT
Record Value: v=DMARC1; p=none;
Host Record: @
Record Type: TXT
Record Value: v=spf1 include.mail.qq.com ~all
After changing the above record values for testing, the email was sent successfully.
For reference.