Add a few alternative providers to Fastmail

We like Fastmail, but there are certaily many other options out there.
List several of them.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

source/email-hosting.rst

diff --git a/source/email-hosting.rst b/source/email-hosting.rst
index 2c82d7a..04ae3ed 100644
--- a/source/email-hosting.rst
+++ b/source/email-hosting.rst
@@ -34,6 +34,12 @@
 - Their server stack is largely proprietary, though they open-source parts of
   their software
 
+**Alternative providers:**
+
+- https://mailbox.org/ (EU hosted)
+- https://migadu.com/ (EU hosted)
+- https://purelymail.com/
+
 linux.dev hosting
 -----------------
 The Linux Foundation offers linux.dev mailbox hosting for anyone listed in the