Good Help - especially when it comes to email marketing services - is hard to find.
Consider MailerLite for your email marketing for your Saas.
Since I'm starting FreelancerCashFlow, I'm going to need a reliable email marketing service. I did my due diligence - checking out the list of usual suspects: Mailchimp, GetResponse, Drip, ConvertKit, SendInBlue, etc. etc.
My needs are simple: excellent deliverability, a generous free plan, support for RSS campaigns so I can use that for Remote or Else, and some semblance of support. Landing pages would be a plus too.
The problem with most services you'll find reviews of is their customer support is terrible ("Check our forums!"), so reachable tech support mattered.
After spending way more time than I should have on this, I've settled on MailerLite. MailerLite is not as well-known as say MailChimp, but they get the job done well.
You can read a good recent review of MailerLite in detail over on WPCrafter.
Their pricing is up to 1,000 subscribers and 12,000 emails a month free, then up from their, but they undercut the other services if/when you get say 10k subscribers.
What has stood out during their free 15 day trial is that their tech support (accessable via an Intercom widget) was both knowledgeable and friendly. When you're setting up a new service for your saas, you are going to need tech support at the beginning.
So, much like SiteGround for WP hosting, my recommendation goes MailerLite a the company that understands that reachable tech support is really necessary and a major product differentiator.