I found when I tried out using SES, Sendgrid, Brevo, etc to send emails for my self-hosted exim instance, that my deliverability actually went down compared to sending the mail directly. I believe it was because for the free/low cost tiers they dump you onto IP addresses where they're also playing wack-a-mole with spammers. SMTP2GO was the only one I found that had decent deliverability at a free/low cost for low volumes. Still, I only use it for my personal email. For my business, I host our mail on Fastmail.