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Just letting everyone know, I just signed up for sendgrid and configured it with ease using their postfix tutorial - straightforward.
Please note: the shared IP I was given on a free account was listed with Sorbs for last 28 days, Yahoo and Gmail differed the delivery.
Whats the point in a paid service if IPs are not kept clean and properly warmed-up? Any experiences?
How about email, are they going through to inbox? Paid plans might have clean IPs there. If you can spare a small VPS you can setup CyberPanel and send as many emails 
More details on getting better email score: Achieve 10/10 Email score with CyberPanel! - Blog Posts - CyberPanel Community
If you properly follow that tutorial I guess you don’t need to use any third party email service, and there will be no limits to how much emails you can send (except your provider limits you)
PS: MailGun also have IPs listed on two blacklists, and mail was not going through to yahoo.
There should be no difference in paid and free as the free plan eventually forces upgrade, ideally the free plan should be more robust as it is a trial account for proof of concept / service.
Mails to Yahoo were not delivered.
I exactly did what you suggested, a $5 droplet on DO + Cyberpanel + Whatever advised in the link. Its working for now, fingers crossed.
Its so stupid of Yahoo to not accept any genuine de-listing requests despite repeated attempts. Gmail on the other hand takes mail to spam (acceptable), max throttles it (can live with that), but not accept mails altogether because the IP was not warmed up? That is sheer nonsense.
In the whole process my clients asked around 50-75 customers everyday for last 10 days to provide alternate address and eventually letting Yahoo customer know that the problem = Yahoo, Is Yahoo on a suicide mission? It would still have been understandable if Yahoo had a paid email service like G-Suite but they like to spam user’s screen with filthy adverts and reject genuine email. Bonkers!!
A lot of work has been done on the email side as far as CyberPanel is concerned, but there are still many things that need to be taken care of for better email deliverability. I was using DO Instance with CyberPanel for all the transactional emails (forum, blog, help desk). IP was clean but the email was not accepted by outlook (the same case, not even going to spam).
Then due to time limitations of getting the IPs whitelisted or accepted at mail providers, I went with MailGun, I recently discovered that MailGun was not delivering to Yahoo, I contacted their support and they were kind enough to replace the IP quickly even though the IP is blacklisted (only at one place) emails are still going through.
So long story short, if you can go through the trouble of cleaning your IPs, self-host setup will be best, you can setup CyberPanel and enjoy unlimited emails.
At the end of the day, even a single email campaign can put IPs on blacklist and effect email deliveries.
Update: 4 hours of genuine business usage less than 100 emails max, Outlook and Cloudmark has blocked the IP - Reputation Issues