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Re: [ietf-smtp] my perspective: viewing SMTP specifications and practice through an IIoT lens

2019-12-23 13:09:12
On Mon 23/Dec/2019 00:13:46 +0100 Keith Moore wrote:
Not to single out any person in particular, but when people make blanket
statements like "real MTAs use DNS names in EHLO" I see a red flag.   It
strikes me as very common for people to assume that the use cases that they 
most are "real", and that other use cases are "not real" or rare exceptions to
the general case.   Use of IIoT devices is expanding rapidly, and we could 
see the number of IIoT devices in use rival, say, the number of PCs in use.

I believe the sentence "real MTAs use DNS names in EHLO" refers to SMTP servers
when they act as clients.  Rfc5321 has a somewhat wider concept of MTA, so you
may claim an IIoT device is an MTA when it acts as a submission client.

Anyway, real SMTP clients use DNS.  I'd consider IIoT devices as submission
clients.  Consider this snippet of the draft standard:

   Many mail-sending clients exist, especially in conjunction with
   facilities that receive mail via POP3 or IMAP, that have limited
   capability to support some of the requirements of this specification,
   such as the ability to queue messages for subsequent delivery
   attempts.  For these clients, it is common practice to make private
   arrangements to send all messages to a single server for processing
   and subsequent distribution.  SMTP, as specified here, is not ideally
   suited for this role.  A standardized mail submission protocol has
   been developed that is gradually superseding practices based on SMTP
   (see RFC 4409 [18]).  In any event, because these arrangements are
   private and fall outside the scope of this specification, they are
   not described here.


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