The STARTTLS vulnerability to downgrade attacks is a significant downside that port 465 doesn't have (if using modern TLS protocols at least). My opinion is that perhaps the severity of the STARTTLS downgrade attack wasn't that much considered when port 465 was deprecated.

Downgrade attacks are no easier for STARTTLS than for TLS-on-connect. If you were deploying TLS-on-connect you'd have to do it in one of two ways:

1. Senders should first try TLS-on-connect, and if it's not available, fall back to STARTTLS.

2. Senders should only do TLS-on-connect with no fallback

If you do #1, then a downgrade attack still exists because an attacker can just block port 465 and then interfere with the traffic on port 25 (or put a fake SMTP server on that port if one doesn't exist).

If you do #2, that's no easier than just mandating that STARTTLS on port 25 is required.

The alternative to all of this is to find some out of band method for a receiver to state that TLS should be available (TLSA / MTA-STS), and if it's not then something is being tampered with. That way, a sender can look up that information and then choose to do #2 for that particular receiver. Again, in that scenario it makes no difference if we're talking TLS-on-connect or STARTTLS. Given we can't make the whole World switch to "must always be encrypted" at the same time, this is the only practical way forward.

I wrote "It has exactly the same upside / downside" and yet almost immediately I get a reply in which the author with no self-reflection has imagined there are additional downsides to STARTTLS.

As others have explained an on-path adversary who'd be able to rewrite packets in order to stop STARTTLS can even more trivially block port 465 altogether. In fact cheap appliances can do the latter but can't do the former, so if anything STARTTLS very slightly improves this.

If you are imagining "Well I could have a policy of only using port 465" well, sure, but you could just as easily have a policy of requiring STARTTLS. In both cases an adversary can deny you, and your only options are to re-think your policy or give up and not deliver email.

The only (weak) argument I can find is that since servers that had a port 465 open also had to keep a port 25 open for compatibility, you could perform the same downgrade attack by blocking connections to port 465, which would result in a submission to the port 25. But that's a "if it's broken let's make sure it remains broken" kind of argument.

Yes - it's a weak argument, and one that's probably been debunked by looking the way https lifted off recently. My view is if port 465 was still around today, it would probably get the same level of attention as port 443 has. We could have been at a stage where port 25 could be made intentionally unavailable (same way we move browsers from http to https) and everything forced to 465. Email agent developers would be forced to update their practices as well, no email should be sent over plaintext. At present, there is no good way to tell your clients you're not accepting plaintext. STARTTS is from a world where 99% of emails were plaintext.

You've confused the submission port (which is usually TLS encrypted on 587) with SMTP relay (done via port 25 with optional STARTTLS)

It's OK, lots of people don't know about this. Really the only problem is that you've assumed since you know nothing about this you're an expert, that'd be Dunning-Kruger or a related effect at work.

For submission it really can work to upgrade end user mail agent software. We're a long way from useful encryption capable agents being ubiquitous, but we've made progress and most popular systems today ship with such a client. These systems also use TLS directly for IMAP (to receive email) and can share a lot of logic across the two systems, so that's nice. In maybe a decade we'll be where HTTPS is today. For submission.

What distinguishes a submission port is that it's authenticated. You can only send email via the submission server as Alice if you prove to the submission server that you're Alice.

But SMTPS (obsolete SMTP over TLS on port 465) and the STARTTLS SMTP (on port 25) are for email relay. Authentication makes no sense in this scenario because the relays are neither sender nor recipient, the transmitting relay can't prove that it's Alice, and the receiving relay can't prove that it is Bob, because they're not, they are merely relays.

These are servers, so they upgrade much more slowly (often only on hardware replacement) and the priority is reliability of delivery, not security, if your proposed solution drops 1% of email on the floor that's far too much and nobody would adopt it.

Hence the situation we have today, and hence STARTTLS Everywhere. Your imagined alternate universe makes no sense except because you've misunderstood how email works.