55 persons using an e3 license would be 15k/y. 55 would definitely fall into my definition of small business
As a solo dev 5 people would feel huge. Going by OECD small is < 50, medium is < 250 and micro is less than < 10.
Small business, a legal designation in the US, can be surprisingly big: Depending on industry, you can have up to 1,500 employees, or $50mm in revenue, and still be considered a small business by the federal government.
We do a lot of opt-in email for the newsletters we run -- most software (such as MailChimp) that provides the services we need is very expensive, especially if you have more than 100k subscribers. Hence, hosting the email ourselves saves a siginificant amount of money, though at the risk of not being particularly user friendly and also requiring expertise in maintaining the server and email deliverability.
Maybe they're a spam business? $15k a year spend at AWS will get you the ability to send 150 million emails.