Interesting; I am quite hesitant to move from Protonmail to a selfhosted mail. How long have you been selfhosting your mail? How do you deal with spam? Are your mails always received at the other end?
I moved to selfhosting email about 2 years ago from google gsuite. I use mailcow[1] and have it on Hetzner Cloud. I don't send much but they have always gotten to their destination. Spam is very minimal, next to zero. Mailcow has built in spam detection with rspamd and lets you train it if any spam does get through just by moving the email to your junk folder.
Another big thing I love about mailcow is that it has sync jobs so I can create mailbox's within it and have it sync my emails from other gmail accounts I have or really any mail host that you can connect to with IMAP. You can even set it so it deletes the email on the source account, which is great as most of my extra gmail accounts are just used to receive.
The dashboard is pretty great too and you only really need to use the shell, apart from the initial install, to update it every now and then. So many great things I have to say about it and how you can super power it like hosting custom domains and nextcloud among other things. I'm glad I made the change to selfhost. If you know what you're doing I'd 100% recommend you test out running your own mail server for a bit to test the waters.
Late reply but it's around 15 euro. I'm using the CPX31 on Hetzner Cloud[1]. It holds my main setup and the extras like nextcloud, custom domains, etc.
I'm self hosting with mail-in-a-box as well. It comes with a nextcloud install. It hasn't been flawless but it's been good and I'm happy to have some control.
With e-mail, I haven't had any problems with one exception. When e-mailing the local school system, they reject my e-mails. I looked into it and it turns out that their spam provider was blocking me because I was a private domain or something like that. It was a configuration on their side. Their tech support told me that I should "get an e-mail address with a normal extension."
Outside of that issue, my e-mails have gotten delivered. Between graylisting and the built-in spam filter software, I haven't had any spam issues. It's been smooth as far as that goes. The webmail (roundcube) isn't as nice as gmail but desktop and mobile clients are good in any event.
The mail-in-a-box nextcloud install does use sqlite which means that you should make sure to backup contacts in case sqlite breaks. It broke for me once but I was able to copy my contacts from Thunderbird back into the system without any real problems.
Calendaring works pretty well with Nextcloud but I haven't found any calendar software that I really love. The web software is good but not super fast. Lightning has gotten better but still feels bolted on. Kontact calendar is too groupware-oriented for my personal use. Evolution never quite felt right to me. The built-in Apple calendar and Samsung (Andriod) calendar apps work fairly well.
I’ve been running MIAB (mailinabox) for business and personal since 2016. I haven’t seen any major issues with deliverability but YMMV. You have to register with all the feedback loops. Once you do, you get feedback when something you send gets marked as spam. Spam on MIAB is handled with postgrey and with spamassasin. It’s pretty good, although I’ve had recent issues with spam coming from gmail and hotmail. I’ve also customized some of the config to bounce Microsoft Sharepoint and Google Docs since neither companies control spam coming from their networks.
I self-host on a small-ish VPS. I don't really have to deal with spam, mail-in-a-box comes with pre-configured spamassassin. I haven't had issues with deliverability. When I created the box I did a scan of the IP and domain, found out that one antispam provider was blocking my IP, I sent a request to unblock it explaining this is my new IP I just acquired, they unblocked it. That was the last time I had to deal with deliverability issues, in any case I have an automated warning that I set up on MxToolBox. Let me know if you have more questions.
I never had any issues with either NextCloud or Mail-in-a-Box, I'm pretty happy about those.