Tips for startups:
Zoho offers 50 free seats for email, with a free custom domain name. It has one of the best web interfaces, and can of course be accessed via Outlook, or any other IMAP / POP client.
Google Cloud Platform offers Firebase, with its latest incarnation, Firestore, having greatly reduced costs, and importantly, can be used for user authentication without spending a ridiculous amount of money on overpriced and aggressively overhyped solutions such as Auth0. Firebase is probably the only reasonably priced third-party solution for authentication in an overpriced market. Firestore is very efficient for user profiles. Auth0 charges thousands for what Firebase offers for $25 or less.
You'll see a lot of developer articles that just happen to mention Auth0 and Cloudinary. Look around - they are not the only game in town, and those articles don't mention that they are sponsored content. Be aware that some are white labelled services from the same original provider..
Use the free tiers offered by Cloud providers such as Amazon, Azure, and GCP. You can, for example, run a perfectly usable instance of Mautic for email list management / basic CRM, in AWS, free for a year, that also gives you 66,000 monthly emails for free, with high delivery rates.