Again... I've found the answer myself

This document had some clues: https://sogo.nu/files/docs/SOGo%20Insta … 0Guide.pdf

The only thing that I had to do was this:

Uncomment these lines from SOGo Apache configuration:

ProxyPass /Microsoft-Server-ActiveSync \
 http://127.0.0.1:20000/SOGo/Microsoft-Server-ActiveSync \
 retry=60 connectiontimeout=5 timeout=360

Restart Apache2 and Bam!!!... or almost! I also had to change the username to "user" instead of "user@domain" to authenticate.

Now I'm happy

Both - I see why you're asking. It should be in the documentation, that way it would save much time. Don't get me wrong, docs are fine, it's just that as it happened to me, it may happen to someone else.

Well, if my reply is not clear for you, I'll try to answer in a different way:

1) Yes, I use https and http. So, the uncommented lines could be:

For HTTP:

ProxyPass /Microsoft-Server-ActiveSync \
 http://127.0.0.1:20000/SOGo/Microsoft-Server-ActiveSync \
 retry=60 connectiontimeout=5 timeout=360

For HTTPS:

ProxyPass /Microsoft-Server-ActiveSync \
 https://127.0.0.1:20000/SOGo/Microsoft-Server-ActiveSync \
 retry=60 connectiontimeout=5 timeout=360

2) Yes, I've solved the problem.

3) The root of the problem is that I couldn't make Active Sync to work following your tutorial, something was missing. So, I've  searched for an answer, and discovered that AtiveSync is not enabled by default in iRedMail setup. As I followed your tutorial, it didn't mention that it must be manually enabled and how to do it (or at least where are the documentation explaining how to enable it). The iRedMail tutorials are great, but in this case in special, I missed this information and it took me a while to figure out and to find how to make it work (an entire day, to be honest).

So, this topic is to help anyone who may be experiencing the same as I did, with the problem and the solution.

Regards.