@codeofamor Thanks for the list you provided.
A couple of months ago, maybe even 1 or 2 years ago, I also was searching for a new E-Mail provider. Because IONOS build in features, that they said, should secure their customers. They don't allow sending E-Mails from an Alias anymore. Which hasn't anything to do with securing their customers... Just restricting working configurations. Anyway:
I couldn't find a new Mailprovider that met my requirements. Sadly at that time neither did mailbox.org or posteo.de.
The thing is, that I use aliases at my own domain intensively. Like mastodon@mydomain.com, twitter@ and so on.
In the last years, I build a lot of aliases. And, somehow mailbox.org just restrict using them (WTF? - there is absolutely no(!) need to restrict the use of aliases even in their most expensive packet!). And I thought, even posteo was restricting them, but now I can't find that restriction in their docs anymore. And I can't create an Posteo account to find out their abilities, because I need to pay. There doesn't seem to be a way of a "30 day trial period" or something.
So, long story short: Can't recommend the german mail provider because of some stupid alias restrictions according to their public docs. And, sadly enough, I couldn't find any other provider that would help me with my aliases other than simplelogin.io. Which might be the cheeto-fingered oompah-loompas you mentioned.
BTW. Posteo advertises 12(!) different colors (schemes) for its web interface on the first place of their website. Come on... it's not 1990 anymore. Although they have a "brand new"(?) dark theme, the web interface looks more like very old SquirrelMail at first glance (https://www.squirrelmail.org/screenshots.php)
Sorry, I don't want to rant. Maybe the privacy features and green energy and free bike repair things for their staff outweigh these things... But I'm not 100% sure if they really can outperform on other important layers in E-Mail Communication, too.
Also I don't even got started about other Mailboxes that I manage for my family members. I don't think that any of the two would have the possibility to manage multiple own domains for multiple (other? as in: payed by someone else) mailboxes.
Probably I will start / need to start my own Mailprovider if I had to much time and money.