I think we can close this than.

I referred to the MS-ASCMD documentation, specific:

https://msdn.microsoft.com/en-us/library/gg650877(v=exchg.80).aspx

where the folder of type 4 is called the "default deleted item 
folder". For me this concluded, that such a folder must be present, so 
that the EAS client can always choose beween moving a folder to trash 
and deleting it via FolderDelete.

However, there is also the OPTION "DeletesAsMoves", which you probably 
do not send, if the user has disabled trash and thus there is no 
folder of type 4. I missed that OPTION.

Thanks for your time,
John

Also, there is no "default" trash folder in EAS, so I'm not sure what 
you mean here.

What isn't?  The use_trash preference is an IMP preference, the MUA.   
If the account that ActiveSync is synchronizing with is not using a 
trash folder, why would we indicate to EAS to use one?
Well, in EAS you can either do a DELETEASMOVES or an immediate 
deletion. The former uses and REQUIRES a folder of type 4 to be set, 
the latter does not and indicates to the server to perform an 
immediate expunge.
A FOLDERDELETE command is always permanent in EAS. I.e., it does NOT 
move any items to the trash.

I assume, this is not part of the ActiveSync protocol? From the client 
side, I have to check if the trash folder is there or not, even though 
it is a default one, which should be there?

A follow up question: If the Horde user did enable the "use_trash" 
setting and issues a FolderDelete request, will Horde translate that 
into a "move to trash" or will it obey the delete request?

We explicitly set the trash folder to 4. See 
Horde_Core_ActiveSync_Driver::_getMailFolder()

If you are seeing the trash folder as type 12, this probably means 
that you don't have the "use_trash" pref set in IMP's preferences.

for drafts, send, inbox you use the correct type values, but for trash 
you use 12 and not 4. why?