Outgoing SMTP Servers

Bill Stewart nonobvious at gmail.com
Fri Oct 28 23:21:39 UTC 2011

There are several models for where the MTA lives in an ISP environment
- MTA at customer, connects to destination via Port 25.
- MUA at customer, MTA at ISP, connects to destination via Port 25.
- MTA at customer, ISP transparently forces connection through ISP
MTA, then connects to destination via 25
- MUA at customer, ISP, MTA at email service provider, connects to
destination via port 25.

The MUA-vs-MTA distinction and the MTA-at-ISP model are
_historical_artifacts_, left over from the days of dial ISPs.
- An MTA benefits from having a reliable full-time connection to the
Internet, because it's going to deliver mail to a potentially
unreliable destination and may need to keep trying repeatedly over a
long time, while the MUA only needs to connect to the MTA long enough
to pass the message.
- It's also helpful for the MTA to be colocated with the sender's
mailbox service, and the mailbox service and its domain names also
benefit from fulltime connectivity.
- While dial internet is almost dead, smartphones and wireless laptops
are partially recreating the unreliably-connected computer system, but
they usually use MTAs at an email service provider, not the ISP.
- On the other hand, any desktop computer or laptop, most smartphones,
and many wristwatches have far more computing horsepower and disk
space than the VAX 11/780s that ran early sendmail systems, so they're
perfectly capable of being first-class objects on the Internet and
running MTAs.

I've got a strong preference for ISPs to run a
Block-25-by-default/Enable-when-asked.  As a purist, I'd prefer to
have Internet connections that are actually Internet connections, and
if you want to run email on a Linux box at home or have an Arduino in
your refrigerator email the grocery when you're out of milk, you
should be able to, and if some meddling kid at an ISP wants to block
it, they should get off your lawn.  In practice, of course, somewhere
between 99.9% and 99.999% of all home MTAs aren't Linux boxes or Macs,
they're zombie spambots on home PCs, or occasional driveby wifi
spammers or other pests, and not only should outgoing mail be blocked,
but the user should be notified and the connection should probably be
put into some kind of quarantined access.

But that's for Port 25 - the Port 25 blocking by ISPs has largely
pushed Email Service Providers to use other protocols such as 587 for
mail submission from an MUA to the MTA, or webmail instead, and it's
really bad practice for ISPs to interfere with that.  In some cases
they'll still be sending spam, but that's the MTA's job to filter out,
and if they don't, they'll end up on a bunch of RBLs.  (And generally
they'll be trying to keep their mail clean themselves - if the MTA
providers were spammers, they wouldn't need to go to the trouble of
having actual residential users as customers when they could
mass-produce it cheaper directly.)

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