What may be blocking outgoing emails in Mailmerge? [closed]

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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?