system
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Thanks, actually the mail shield seems to be the reason for the different behaviour. When I disable it, the alert email can’t be delivered either, if TLS is required. (Interestingly it isn’t enough to disable/enable the mail shield - additionally a reboot is necessary)
So in the end it is currently not possible to use virus alerts in conjunction with a mail server that requires TLS before authorization.
Because the latter is considered a secure setup, I would appreciate if it could be supported in a future release.