a) I wouldn't confuse your pricing model with the delivery model here. You can certainly charge monthly for downloadable self-hosted software or charge once for SaaS hosted in the cloud.
b) If you're in email, and you're not working with clients at the very, very top of the sophistication chain, you should not expect them to host their own email servers because their deliverability will be terrible. I suppose you could theoretically let them use your product with a bring-your-own-MSA, but that should probably be in-scope for your product. (You'd also be crazy to do this yourself, but you can use an MSA on the backend, like substantially every company providing UI and logic on top of email does.)
c) The operational difficulties of doing release cycles and maintenance for client-hosted software, particularly hosted software which has to play well with client-provided infrastructure, strongly, strongly, strongly suggest you host things yourself.
b) If you're in email, and you're not working with clients at the very, very top of the sophistication chain, you should not expect them to host their own email servers because their deliverability will be terrible. I suppose you could theoretically let them use your product with a bring-your-own-MSA, but that should probably be in-scope for your product. (You'd also be crazy to do this yourself, but you can use an MSA on the backend, like substantially every company providing UI and logic on top of email does.)
c) The operational difficulties of doing release cycles and maintenance for client-hosted software, particularly hosted software which has to play well with client-provided infrastructure, strongly, strongly, strongly suggest you host things yourself.