Earlier quoted context omitted.
> despite sending an email being an utterly trivial, long solved problem. It is absolutely not. Even if you are not considering the whole spam thing. Sending an email with the multitude of clients, MIME which is very complex in itself, network issues, message queuing, retries, unsubscription, bounces, providers feedback loops, rate limiting and what not is very hard. If you add the whole spam folder + blacklists situ…
Sending an email is absolutely a solved problem. I can send a message to my address in 2020 using a 2003 Fedora Core 1 box without a domain using only what's part of the POSIX standard, and I'll receive it just fine. I just did a few days ago, actually. It even got past my provider's spam filter. I could probably have done it using earlier software than 2003-era mail/mailx, but that was the quickest way I could find…
tl;dr: Abuse ruins it for everyone.
I work on the filtering side of the world so I see a lot of the challenges that even purportedly good actors face. Each provider does better or worse at controlling abuse. These things all have a cost:
* IPs -- You need to send from a variety of IPs. They have warmup time before they can be safely used. One bad guy can burn that IP and everyone who shares. Most ESPs aren't going to have dedicated IPs for each (or those are for high tiers).
* Dmarc/dkim/spf -- Passing validation can be hard since it requires your client to work with you.
* Abuse reports -- Likely huge volume
* Bounces -- They can count against the customer. How much is OK (e.g. AWS SES will cut you off if your bounce rate gets too high)?
* Throwaway account abuse -- Huge issue, this threshold is a simple hammer (so now bad guys need ~10x more throwaway accounts).
* Account takeover -- That good guy is now sending out crap. Now what?
* Post-abuse cleanup -- Good luck going around and working with major providers to get yourself unblocked. Its a huge time sink.
By using Mailgun, etc all of the above becomes their problem. On top of it they'll offer analytics, help crafting content that works in varying mail clients, etc.
A small business type sender doesn't need much of this (other than maybe the technical help). But at some point, the scales tip.