According to USPS regulations and guidelines, you are required to issue a Private Mailbox Number for each virtual customer.
You can easily assign a PMB# using SphereMail by following the steps below:
- Go to the section titled "Customers"
- Go to the user you want to assign a PMB# to
- Click on the # symbol
- Choose a number from the drop down menu
- Click the Assign Mailbox#
You are not required, however, to issue a PMB# for physical customers working or renting an office at your space.
Follow the link for more information, look for section 1.8.2 (points e. f. and g.) : https://pe.usps.com/text/dmm300/508.htm