I tried installing Dovecot from here, but it put my iPhone into a boot loop (try it at your own risk!).
The reason I want it is so that I can distribute arbitrary files to other iPhones using a local WiFi network without sending data to a cloud sharing service. For example, a mixtape of MP3 files, or a photo album.
> AirDrop is an ad-hoc service in Apple Inc.'s macOS and iOS operating systems [...] which enables the transfer of files among supported Macintosh computers and iOS devices over Wi-Fi and Bluetooth, without using mail or a mass storage device.
A publicly-referenced SMTP server MUST NOT require use of the STARTTLS extension in order to deliver mail locally. This rule prevents the STARTTLS extension from damaging the interoperability of the Internet's SMTP infrastructure. A publicly-referenced SMTP server is an SMTP server which runs on port 25 of an Internet host listed in the MX record (or A record if an MX record is not present) for the domain name on the right hand side of an Internet mail address.
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However, this RFC is now 18 years old.
https://datatracker.ietf.org/wg/uta/documents/
shows that https://datatracker.ietf.org/doc/draft-ietf-uta-email-deep/ is now in Last Call -- the title is "Cleartext Considered Obsolete: Use of TLS for Email Submission and Access" and coming up soon we have https://datatracker.ietf.org/doc/draft-ietf-uta-mta-sts/ with this abstract:
SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections, and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate
If the recommendations don't make sense, don't follow them.
In general, I would tend to agree -- so long as you aren't "hurting" (affecting) anyone else.
OTOH:
$ tail -n 4 ~/.signature
"The total budget at all receivers for solving senders' problems is
$0. If you want them to accept your mail and manage it the way you
want, send it the way the spec says to." --John Levine