[horde] Update horde via Debian testing?

[Thomas Trepper]

Hi all, 

I am using horde webmail 5.2.2-1 for Debian 8 (Jessie) (https://packages.debian.org/jessie/php-horde-webmail). 
I have seen, that in testing the package is updated to 5.2.17-1 (https://packages.debian.org/stretch/php-horde-webmail) and I was wondering 

a) why the packages are not available as an update for Debian stable
b) if it is a good idea to update *only* the horde packge via testing to 5.2.17-1?

Thanks a lot in advance for any comments and suggestions. 

All the best and a happy new year,

Thomas