https://gitlab.synchro.net/main/sbbs/-/commit/04ccf2e43bc7889e1b0e0554
Modified Files:
	src/sbbs3/mailsrvr.c
Log Message:
If already SMTP-authenticated, fail subsequent AUTH commands with 503 response
RFC 4954:
          After an AUTH command has been successfully completed, no more
          AUTH commands may be issued in the same session.  After a
          successful AUTH command completes, a server MUST reject any
          further AUTH commands with a 503 reply.
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