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# test plain text port
curl --ssl --url mail.example.com:25

# output returned
#   can indicate that the port is not available:
#     curl: (7) Failed to connect to mail.example.com port 25 after 2186 ms: Connection refused
#   can indicate that the port is available: 
#     220 mail.example.com ESMTP Postfix (Debian/GNU)
#     221 2.7.0 Error: I can break rules, too. Goodbye.

# test SMTP over SSL
curl --ssl --url mail.example.com:465
# output returned can indicate that the port is available:
#   curl: (56) Recv failure: Connection reset by peer

# test SMTP over TLS
curl --ssl --url mail.example.com:587
# output returned can indicate that the port is not available:
#   curl: (7) Failed connect to mail.example.com:587; Connection refused