If your hosting email on a residential ISP, your gonna have a rough time. Comparatively, hosting on any of the notable VPS providers (DO, Linode, OVH, etc) has had fine deliverability in my experience, though emails may occasionally get flagged as spam.
Beyond that, the only insidious behavior I've seen has been Outlook.com hosted addresses accepting email, but then not delivering the email to the Inbox or Spam. Mysteriously, this issue disappears whenever I start going up the IT chain on the reciever's side. If the MTA is going to drop email, it should reject it rather than accept it (as otherwise the Outlook MTAs are falsely accepting mail that they will not deliver).
IIRC though DO tries to make sure a released IP is returned back to you if you re-provision quickly, you run the risk of losing your well groomed IP for outbound mail if there is an issue with your droplet.
Additionally, IIRC they forbid outbound SMTP on their Floating IP system. (I can't find an official doc on this matter). Either way, you can't get a PTR for your floating IP, which can harm deliverability[1]
For that reason, you may want to consider AWS & an Elastic IP that you can own, groom, and move around [2]
Furthermore, DigitalOcean silently drops all SMTP traffic on IPv6, which can be really confusing since you'll simply timeout and it isn't mentioned in the documentation. They do that because they implemented IPv6 against all standard industry practices (which were helpfully gathered in a RFC) and within hours of launching IPv6 the few /64s from which they were allocating customers /124s to landed on blacklists.
It's been 5 years since and they still have no plans to do IPv6 properly and allocate /64s [0].
This is the main reason I switched to Linode, who will happily allocate you a IPv6 range (/116, /64, /56) that can be rerouted between VPSes with a simple ticket (takes <1 hour).
Don't destroy your droplet then? Digital Ocean supports reinstalling VMs in place without destroying the VM. Floating IPs for a mailserver (esp. outbound) sounds a bit silly, mail infrastructure is more fault tolerant than other systems, to the point that a minute or two of downtime is far from the end of the world.
Receiving email is no issue on residential ISP.only issue is sending, And to get around that you simply use an external SMTP server and your problem is mostly solved.
It's just something you have to do anyway since more ISP prevent sending directly from home connections. That means it will cost some money, but there are also SMTP services with free tiers for several thousands of emails per month.
> hosting on any of the notable VPS providers (DO, Linode, OVH, etc) has had fine deliverability in my experience, though emails may occasionally get flagged as spam.
I can share a polar opposite experience: Yahoo and Outlook would categorically reject anything DO at the SMTP level with a 502. Luckily enough, at least they were explicit about it. But no leeway, not even accepting the envelope body.
I don't run my mailserver on DO (I know a few people who do tho), but I've had few issues operating one on a no name host and another on OVH. If that was a persistent issue, it might be advisable to move to a host that isn't blacklisted.
Sendgrid is usually spam or redundant notifications, depending on host they might not make it through without manual whitelisting. Additionally, using a totally unrelated SMTP rela to deliver mail defeats much of the point of a self-hosted mailserver, you'd be better off using your local network's SMTP relay.
You control the way you authenticate users, how to run spam filtering, etc.
I run my own on Linode with postfix/dovecot/rspamd linked to LDAP for auth that gets routed through SES (probably some cents per month) and it's working good.
These days it's not easy to circumvent being flagged as spam running your own node, because you can look like any spammer until you prove yourself you're not one which takes time and usually not the best idea and you'd rather use an external relay who will provide you with a better reputation from day one.
If you stick with your own delivery route and friends tell you, your mail isn't arriving as they get flagged as spam, that's your fault.
Beyond that, the only insidious behavior I've seen has been Outlook.com hosted addresses accepting email, but then not delivering the email to the Inbox or Spam. Mysteriously, this issue disappears whenever I start going up the IT chain on the reciever's side. If the MTA is going to drop email, it should reject it rather than accept it (as otherwise the Outlook MTAs are falsely accepting mail that they will not deliver).