Optionally limit concurrent connections to mail server
The mail (SMTP) server is a popular target of bots. Impose a maximum-concurrent-connections limit (optionally). Similar to the terminal server option, except, don't deduct the number of authenticated connections (I can add something like that if desired). The new config option is [mail] MaxConcurrentConnections in the ctrl/sbbs.ini (defaults to 0, no maximum). Fixed error responses in mail_server accept loop: was always sending POP3 error responses even if the connection was SMTP. Cleaned-up the service/port checking/logging in the mail_server accept loop: no strcmp() needed, log the protocol name (e.g. "SMTPS") instead of the service name (e.g. "submissions").
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