The difference between what? different providers? DIY or outsourced? I use outsourced because deliverability should be their number 1 concern. They handle all the opt out/opt in hassles. When I've done it DIY I've usually come unstuck with SMTP servers having throttle turned on - and even when it's off it somehow manages to prevent the emails being sent. For a small fee I can reduce the stress and hassle. Its well worth it.
The thing is, people use specialized SMTP services because they want to avoid their email going straight into the spam folder as little as possible. Besides what's written within the email, most email providers look at a persons' reputation when it comes to ip-address, internet provider, & SMTP service provider.. when making a decision about spam. So to increase proper delivery & success, companies rely on these services.
In fact, you can find many of them, for example you could make use of SMTP.com or sendgrid.com. I have affiliate relationship with SMTP.com and as far as I understand they're the only ones who will speak to you before you become their customer there more on the consultant side of business