We are actually in process of shifting away from this software. I can not really think of any pros to talk about when it comes to this solution that make it better than something else. All I have to mention are cons because this is pretty outdated software. As proof of how outdated it is, even IBM — the creator of this email client — sold it to HCL to get rid of it.  

It is pretty much outdated and there are no not many pros of using it. Although, if you have to count, integrations are easy. My company has integrated a lot of different systems into HCL Notes. We have systems like leave (vacation) management, we have systems like resource management, and internal files and documents. All of these have been integrated with Notes. So it is a tightly knit tool. Really there are so many integrations that there are far too many.  

We have not done any comprehensive search to see if there are other email clients that allow this type of integration. For example, I would not know if even something like Microsoft Office allows users to build integrations or your own features into their email client. HCL Notes sure does allow integrations of many kinds and that is maybe the only pro that I currently find in this solution.  

Its ongoing license cost is a fraction of its competition. 

Its security is absolutely stratospheric. In an application, you can lock down to field-level security. For example, I could log in and see a field in an HR database with salaries. When you log in, you see exactly the same database, but the field with the salary just isn't there. So, you can literally have field-level security on it, and you can also have secure encryption. For example, I could give you that database allowing you to see the salary, but a calculation that shows how much they get per month in another field is encrypted. So, you won't be able to see that because you don't have the encryption key. You can implement high-level security very simply. 

The most valuable feature is the UI.

It is easy to manage. We have experience with HCL Notes, that is why we prefer it. I personally have 20 years of experience using it. 

The solution is practical, good interface, and performs well.

The usability of the old applications to the new version of the client is very good. The migration is very good for some legacy applications.

Security is the most valuable feature, better than you get in 365. It's a great system.