It's not even a matter of raw volume. The system I'm looking after sends just short of a thousand external emails a day but none of those are marketing or email blasts; it's all people mailing other people and reports being sent to specific addresses that have requested them.
The occasional marketing type bulk email is always sent through an external company. This means I never have to worry about getting blacklisted by those messages.
Note that the "addresses that requested them" out has a number of problems. I've worked for any number of shops that had quite putatively legitimate notification emails that they sent out regularly. Which at one point some person had double-opt-in confirmed....
... but for which there was clearly no valid reason for the mails being sent any more. As in, "the domain expired five years ago and is now a block hole", or "AT&T retired that domain a decade ago and migrated all its users elsewhere", or "that former email provider has had its domain parked on some Russian server for years".
If you're not periodically revalidating your email lists, checking for retired domains, and/or processing bounces properly, you are sending spam, I can guarantee it. Quite possibly not spam that anyone particularly cares about, but it's creating traffic for somebody's email system (starting with your own).
The occasional marketing type bulk email is always sent through an external company. This means I never have to worry about getting blacklisted by those messages.