Cannot send emails

Suddenly I can’t send emails any more. All I get is the following error message

Transaction failed. The server response was: smtp691.mel.oss-core.net
has anyone else had this problem?

Please read and furnish the information listed in this Guide: https://www.manager.io/guides/9100.

Had this problem for some weeks now, I found it as my modem firewall set to high security. I changed it to low and emails started working again. My server message was a time out one!!

Rather than reduce your firewall security from high to low, you might like to investigate giving the required permissions to the email service