Here I would propose enterprise software. As a marketing buzzword (buzzphrase?) it admittedly has become rather inexact. Nevertheless, it encapsulates what the three examples have in common, aside from their manufacturer: they are "heavy-duty" software products that the typical end user does not interact with directly; they are useful (or cost-effective) only at the enterprise level (i.e. for a large or complex organization). An end user would almost never see screens for managing Exchange, for example. He or she would likely interact with Outlook, and Outlook communicates with and through Exchange. He or she would almost never write or extract data directly from SQL Server, but use another application or website to do so.
Business solutions software is somewhat orthogonal to "enterprise software," since business software would include software aimed at businesses which are too small or simple to require products as heavy as SharePoint or Exchange, and strictly speaking a product like SQL server might be used for production as opposed to business purposes.
I would not use the term productivity software here as it is the least precise. To some extent, all software that is not specifically intended for entertainment (i.e. to kill time) would contribute to productivity (i.e. to save time), including the packages mentioned, but the term is applied to applications used commonly by a variety of workers in a variety of industries to enhance their day-to-day productivity. These would include general office software (e.g. word processors, contact managers) and role- or industry-specific software (e.g. accounting, process management). My smartphone app store has a "productivity" category but it certainly wouldn't be the place to buy SQL Server.
Middleware refers specifically to software that manages interaction between systems. Neither SharePoint nor SQL Server are middleware.