ESPs (Email Service Providers) come in a very LARGE range in qualities, as do inbox providers. As with any outsourced service you must rely on the assumption that they will quickly mitigate abuse to their networks, understand the product they are selling (email delivery) and provide you with a better service than you can provide yourself. Since the cost of delivering email is much lower than the cost of delivering physical mail.
A few things you can do.
- Ensure the provider offers DMARC Compliant email, (SPF and DKIM)
- Require you to provide your own non-purchased list (for newsletters)
- Try them out, you can easily send some email via one provider and your in house solution.
- Check your own inbox, what providers do the newsletters you see use.
- Read the AUP
- Request a dedicated IP address (good if you have consistent volume, bad if you don't, helps fight noisey neighbors.
If you have some crappy WordPress on DO, any ESP will be better than trying to setup mail servers in someone else's data center on someone else's IP address. If you have infrastructure, and software that can handle bounces, abuse notifications, delists, blacklists, whitelist, feedback loops... Then do that. Either way it's your domain and your problem :). Self promotion I talk more on this on a podcast linked here
TLDR; In my opinion, no good reason not to use a relay, even if chained with other solutions like data loss prevention or secure email systems (Zix) depending on the sensitivity of the data you send in email.... Don't send sensitive data in email.