FSF compliant Raspberry Pi alternative?

> what happens to emails I receive if the BeagleBoard is switched off for a
> few minutes?

Nothing at all. When other mail servers can't connect to you, if they're properly configured, will put the message on hold and try again later. How often it re-attempts delivery and for how long it continues to re-attempt delivery depends on how the other person's mail server is configured.

Trisquel is only x86. You can try Debian ARM with main repository (free software only).

> I am thinking about buying a BeagleBoard because it seems to be FSF
> compliant other than the GPU which requires nonfree blobs to work.
> That's not really a problem for me because I think the CPU would be
> enough for what I want to use it for.

I'm too looking for a single board computer to use as a small server,
for distro porting and some embedded purposes. I think BeagleBone Black
is ok for these, while there are more expensive boards with AllWinner or
iMX.6 SoCs, some have a SATA port which is useful for servers storing
more data (some have also wifi requiring blobs, it's not useful). There
is much more work done on free GPU drivers for them and it's probably
easier than for PowerVR (which BeagleBoard/BeagleBone and AllWinner A31
have).

(Exynos and Broadcom SoCs need nonfree software to boot, only one
popular SBC uses one of these.)

> Anyway, I am thinking about using a BeagleBoard as a personal mail server.
>
> Assuming this is possible, i.e. use a BeagleBoard with a lightweight
> GNU/Linux OS (probably use Trisquel without a graphical front end) (if
> it's not, please let me know), what happens to emails I receive if the
> BeagleBoard is switched off for a few minutes?

There is no ARM port of Trisquel yet, while it should be easy to do
(both Debian and Ubuntu support ARMv7; the Debian port is named armhf).

> What I mean is, if I switch the BeagleBoard off for 1 hour, and people
> send me emails during that 1 hour. Are those emails lost forever or
> will I end up getting those emails when the BeagleBoard is switched on
> again?

Proper MTAs resend the mail in case of temporary delivery errors
(i.e. not when the recipient MTA tells it to not retry). It works and
is used in greylisting to prevent receiving spam: non-zombie mail
servers will retry the mail after at least five minutes. Some retry
after a minute, some after 15 minutes, they continue retrying less often
for several days.

When my mail server was off for several hours, I received the mails
with a several hour delay. It shouldn't be an issue for proper email
use.

This is something along the line we might be working on in the future. ie small device that can act as a mail server, etc but more privacy friendly. Think additional legal protections from having mail stored locally as well as VPN (ie to bypass SPAM filters and enhance privacy). You can't easily get a judge to order a provider to reveal keys that impact everybody if there is no provider or single point to tap. Right now it's a pipe dream, at least until we get our new web site finished (yea- it was suppose to have been finished 2-3 times now, but due to slow progress, new developers, and really bad things happening in a really untimely way it hasn't been completed).