This answer depends in part on the applicable labor laws in your state, province, or country. You should seek guidance about them and any relevant judgments before making your final decision on matters like these.

In general, a union's role is to represent the interests of its members and to communicate with those members. That communication includes sharing information with them, and being accessible to them in case employees have questions or problems. It is relatively easy to do this when everyone works at a central work location, but more difficult when the union members are dispersed.

While there is considerable debate today about how much privacy exists in an employer's e-mail system, the consensus seems to be that e-mail on a corporate network is not necessarily private. This means that union members who use e-mail to communicate with their representatives (or vice versa) cannot be assured these are confidential communications. In addition, companies have their own policies about how and when unions can communicate with their employee members.

Apart from these legal and policy issues, the main point here is how to best allow, or enable, a union to stay in touch with its members who are telecommuting or elsewhere off-site. The simplest answer is for the union to set up its own e-mail network accessible only to its members. This avoids all the complications of corporate policies, but makes matters more complicated for people who now need to check an additional system for messages. This inconvenience might be a small price to pay if the union wants to avoid the constraints and privacy concerns about using the employer's system.