Alan DeKok wrote:
That, alone, says that the vast majority of spam is abusive of SMTP,
as SMTP is not a one-way message delivery protocol.
IMHO SMTP is a "move the message closer to the recipient mailbox"
protocol. On the SMTP level there are no such thing like one-time
one-way or two-way. (MTA is comparable to a router. And non-firewall
routers does not know about sessions.)
And something about the subject:
SMTP compared to other protocols is more attackable because there are no
rules assigned to email addresses while there are rules assigned to
ports. (For example nobody can use a DNS service as an NTP service.)
z2
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