What may be blocking outgoing emails in Mailmerge?

The first time I used Mailmerge email it worked perfectly. Since then outgoing messages will not send. They appear to start sending, but nothing is actually sent or received. I am using Windows10 64 bit, the latest version of Libre Office 64 bit, and AVG free antivirus. I am trying to send through Gmail, and when I test the settings using either Port 25 or Port 587, connection is established with the server. My ISP provider is Virgin Media. Something seems to be blocking the outgoing e-mails. Any suggestions please?

Similar unsolved Pb, on Windows 10 64 bit, since the last vorking LibO version
E-mailing procedure is blocked at step 4 (befor merging ocurs)
I face that behavior up to Version 5.2.6 (have to try with 5.2.7)
So I always have to come back to the working version !
It’s a pity.