One objection I hear regularly from business who are using their emails incorrectly is that Exchange Online ONLY allows email to 1,500 recipients a day. Many use that as an excuse for staying on premise because they want to bulk email from their own inbox.
Firstly, Microsoft has just raised that limit to 10,000 recipients a day from 1,500.
Secondly, you really shouldn’t be using your normal email accounts for bulk email. A much better bet is something like Mailchimp, but if you must (and some MUST) then ensure you follow the guidance provided by Microsoft when it comes to bulk emails and you’ll never reach those limits!