They've been around for three decades: they MUST be doing something right!
I've been using Mail.com for one of my accounts since 2008. While naturally everybody's experience is different, I always found their service to be rock-solid, trustful, and reliable: they do store your emails encrypted and do not sell any of your data for profiling or for scrapping content to train AIs, and they don't 'suddenly' change their terms of service to 'unexpectedly' consider your account to be ineligible – and kick you out. That simply doesn't happen on Mail.com. I've actually joined them because, for one of my personal accounts, I didn't want Google to scrap all my very personal data – back in the day when it was only used for profiling. Today, who knows what else they do. I still have a Google account, of course – it's unavoidable – but I sleep safe every night knowing that at least the email address with Mail.com (and you can have ten different ones or so) will *never* 'disappear' for an unknown reason – no surprises there. Ever. Granted, the interface is dated – the last change was in the mid-2010s, if my memory doesn't fail me – and the feature set is reduced (but functional), but at least it's quite fast. I would certainly love being able to have, at least, a Dark Mode, and perhaps more choices of fonts and so forth. In fact, I'd be happy if I had all the features present on a popular open-source frontend such as Roundcube (with the Elastic skin). That said, I almost exclusively use a dedicated email application to access the mailbox using IMAP, so it's a bit irrelevant. Mail.com also offers some cloud-based storage associated with your email address, possibly as another way of getting some income (you only get 2 GB for free) besides ads. I haven't used them much, except for some simple experiments; granted, Google, Microsoft, Apple, and even Yahoo offer so much more, also for free, but at the cost of getting all your emails read by some algorithm...