Two different types with different rules and different infrastructure.
TRANSACTIONAL
Order confirmations, password resets, receipts, dispatch notices, appointment reminders. Sent to one person in response to something they did.
These are expected, opened, and generally welcome. They can be sent from hosting mail provided authentication is correct, though a dedicated provider handles them more reliably at volume.
MARKETING
Newsletters, offers, campaigns. Sent to many people at once.
These need consent, an unsubscribe link, and a dedicated sending provider. Sending them through hosting breaches sending limits and damages the shared reputation.
WHY THE DISTINCTION MATTERS
Mixing them damages both. If your marketing produces complaints and both go out from the same address, your order confirmations start landing in spam too.
THE PRACTICAL SETUP
Transactional mail from a noreply@ or orders@ address on your domain, sent through authenticated SMTP. Marketing from a separate address, through a marketing platform, with its own authentication records.
Both need SPF and DKIM configured for their sending source.
