An Exchange Service account provides the login credentials and access for the PoliteMail Server to talk to Exchange EWS to expand dynamic distribution lists.
The service account can be set-up either with Exchange Management console or using Active Directory Users and Computers controls.
No admin permissions are required for this mailbox, this service account mailbox is a user (K1 level in Office365) level mailbox (email address and password), which is only utilized by the PoliteMail Server for authentication with EWS when expanding membership of distribution groups. The mailbox name is not important and will not be used for any send from address, most customers use email@example.com.
The service account credentials are input into the PoliteMail Server Manager > Exchange Web Services control page, and will be stored encrypted with AES-256.
If you will be sending to membership protected lists (groups with membership hidden), then the service account must be added to the security group with permissions to view membership in order to expand the list.
Please note, there is a delay between the time the account is created, and the time it becomes available to the system. In other words, if you test the expand DL option within the Server Manager within minutes of establishing the new account, it may not work right away. Give it ten or fifteen minutes and try again.