I have been a customer at Mailbox for about five years now and no complaints as they have been really reliable in delivering and sending e-mail because it is so affordable i kept my subscription active even when i use others as my main and because Mailbox is not a free service so unpaid accounts get deleted after 30 days, iirc.
The main UI received a update this past year so it is better now but the admin settings where you manage the account really needs a refresh. Badly. The settings there are unintuitive to put it mildy.
Anyway today after i noticed that my calendar and contacts stopped syncing several weeks back for some unknown reason and me spending more time than i care to admit diagnosing it i also ended up registering on Posteo just to see how their ui and ux is and i do like it but mainly the settings are well laid out and quick to navigate through.
The advantage with Mailbox is active/exchange sync which Posteo lacks
but this stopped working for me for some reason so my calendar and
contacts was out of date on Android. On my iPhone everything still syncs
and iOS even has built in support for Cal/CardDAV so no extra apps
needed. But i ended up having to set it up using DAVx5 which is what Posteo also
requires for Cal/CardDAV syncing. I am suspecting it was the Mailbox
rollout of their new 2FA that caused active sync to stop completely.
And Mailbox is definitely geared more towards those looking
for a Office 365 and Google Suite replacement as they offer quite a set
of features. And custom domain is also something Mailbox offers on their
higher tier plan that Posteo lacks but again the Mailbox web ui is
unintuitive to use for managing settings and aliases related to the
custom domain.
When Thunderbird announced
they were working on a mail service i signed up for the beta straight
away. Fingers crossed for this becoming something solid with custom
domain support.
Rant over.