Mailchimp (Intuit) is the incumbent small-business ESP. Transactional sending is via Mandrill (a paid add-on); it has a REST API, webhooks, and integrates with nearly everything.
Use MailKite for transactional and Mailchimp for marketing, or migrate inbound to MailKite. See /docs/integrations/mailchimp.
Read the full setup guideA minimal, copy-paste starter. Swap in your own domain and API key.
# Any app that accepts an SMTP server works with MailKite
MAIL_HOST=smtp.mailkite.dev
MAIL_PORT=587 # STARTTLS (or 465 for TLS)
MAIL_ENCRYPTION=tls
MAIL_USERNAME=mailkite # username is ignored
MAIL_PASSWORD=mk_live_... # your MailKite API key
MAIL_FROM_ADDRESS=you@yourdomain.com # on a verified domain
MAIL_FROM_NAME=MailchimpPoint a domain, set one SMTP endpoint, send your first email in minutes. Unlimited domains, no credit card.