I don’t think it’s the existance of big providers, as much as the general problem of spam, lemmy will likely have this too one day if it grows big, with or without big corporate backed lemmy’s. Fact is, it’s trivial to set up an e-mail server, and have it send millions of spam messages a day to thousands of addresses. You can then register dozens of domain names for a few dollars, and fill the internet with millions of spam messages.
Which is why pretty much all e-mail servers default anything that isn’t known to be throttled (IE a gmail account won’t let you just send as many messages as your bandwidth can handle). A black list whack a mole is basically an unwinnable battle on that front, all anti-spam measures kind of have to start with a “prove you aren’t a spammer then we’ll whitelist you”, rather than the opposite.
But the main point still remains, there are dozens of e-mail providers that have proven they aren’t spam, and more or less ones that meet every overall goal one might have. Ones that don’t track you or put ads (some you may have to pay for, but that’s the options). Still 100x healthier than say facebook and twitter where you consent to all their tracking and rules, or you can’t talk to their members ever.