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.):