I worked for an Email Service Provider for four years. There is a big difference between sending an email and having it get past the spam filter and inboxed.

Just like location is key in real-estate, deliverability is the key when it comes to email marketing.

If the prospect never sees the email - does it matter how cheaply it was sent?

I'd suggest a split test sending a segment of your via multiple senders, and then measuring unique opens & click-thrus.

If it is a purchased list - you'll want to scrub it to make sure your deliverability doesn't tank overnight if you send to a spam trap.

I'd suggest using 2 services at a minimum. 1 Service to send to cold email addresses, and when someone opens or clicks on a link transfer that email to the 2nd service. You may need to replace service 1 frequently if deliverability drops.

That will keep the openers and clickers from getting nuked when if you have a spam trap on your purchased list.