Posteo vs Tutanota vs OpenMailbox

Senor Banana has the point though..

It's not a blanket condemnation of getting your email from an email provider.

I have not pretended it is. Sending/receiving email is not SaaSS. My point only is, well, what I wrote: "You can never have control over a service someone else runs". https://www.gnu.org/philosophy/who-does-that-server-really-serve makes that same point, several times, e.g.:

For the server operator's sake, the programs on the server had better be free; if they are proprietary, their developers/owners have power over the server. That's unfair to the server operator, and doesn't help the server's users at all. But if the programs on the server are free, that doesn't protect the server's users from the effects of SaaSS. These programs liberate the server operator, but not the server's users.

Releasing the server software source code does benefit the community: it enables suitably skilled users to set up similar servers, perhaps changing the software. We recommend using the GNU Affero GPL as the license for programs often used on servers.

But none of these servers would give you control over computing you do on it, unless it's your server (one whose software load you control, regardless of whether the machine is your property).

That said, https://www.gnu.org/philosophy/network-services-arent-free-or-nonfree.html is definitely a better reference. RMS clearly explains there why "the four freedoms that define free software don't make sense for services":

What's clear is that the issues about a service are different from the issues about a program. Thus, for clarity's sake, it is better not to apply the terms “free” and “nonfree” to a service.

Let's suppose a service is implemented using software: the server operator has copies of many programs, and runs them to implement the service. These copies may be free software or not. If the operator developed them and uses them without distributing copies, they are free in a trivial sense since every user (there's only one) has the four freedoms.

If some of them are nonfree, that usually doesn't directly affect users of the service. They are not running those programs; the service operator is running them. In a special situation, these programs can indirectly affect the users of the service: if the service holds private information, users might be concerned that nonfree programs on the server might have back doors allowing someone else to see their data. In effect, nonfree programs on the server require users to trust those programs' developers as well as the service operator. How significant this is in practice depends on the details, including what jobs the nonfree programs do.

However, the one party that is certainly mistreated by the nonfree programs implementing the service is the server operator herself. We don't condemn the server operator for being at the mercy of nonfree software, and we certainly don't boycott her for this. Rather, we are concerned for her freedom, as with any user of nonfree software. Given an opportunity, we try to explain how it curtails her freedom, hoping she will switch to free software.

Conversely, if the service operator runs GNU/Linux or other free software, that's not a virtue that affects you, but rather a benefit for her. We don't praise or thank her for this; rather we felicitate her for making the wise choice.