The Send To commands establish a connection between 3ds Max and Mudbox. While the connection is established, the applications share data. To break the connection, simply close the scene or exit one of the applications.
You must have installed an equivalent version of the other application to see the Send To command options available in the File menu. For example, no Send to Mudbox option is available if you are running 3ds Max 2018 and Mudbox 2019. (If this is the case, use the instructions in Import a model from a different version 3ds Max, below.)
It is recommended to share data between 3ds Max and Mudbox using the Send to Mudbox feature in the 3ds Max File menu. However, if you are attempting to share data between differing versions of the software, for example, 3ds Max 2018 and Mudbox 2019, the Send to Mudbox feature is greyed out. Instead try:
In Mudbox, depending on your requirements, select File > Open or File > Import, and select the model you exported from 3ds Max. You can set options for import in the section. For more information on import options, see Open a model or Import a model.