There are no obvious new user features or functions in MPro 9 although R-Comp has added from performance tweaks and there is clearly lots of code maintenance going on in the background.
Messenger has also been tweaked to give better message handling on other platforms.
Switching better other mailboxes has also been improved.
The big new feature is support for the new AcornSSL module. This can be accessed in the MPro configuration settings. You also have better control on routing email and identifying your computer to avoid having emails falsely identified as spam.
The module is included in the update with clear upgrade instructions. MPro itself is updated simply by dragging the new version over your existing copy.
The guides on how to use MPro with have been updated. It would be nice if all the documentation was reviewed as some is now clearly out of date - I would be very surprised if R-Comp were still negotiating with Castle as mentioned in the readme.
This is a chargeable update and can be obtained from !Store or R-Comp directly.
Overall MPro 9 is a nice to have update which will keep MPro current and I would recommend getting the upgrade (which is 20 pounds). If you want to keep relying on the software and seeing it developed.
It is also probably a good opportunity (whether you upgrade or not) to check your settings as Messenger Pro has an awful lot of features and functionality. I do not believe I am the only person to miss the rather nice addition to use Ram to boost performance added severel releases ago!