Do you want users from the place B to access the mail server through VPN or through their WAN connection?
If it is through VPN, you should be able to with the private IP address and the local ports of the email server.
EMAIL SERVER LOCAL PORTS <--- EMAIL SERVER LOCAL IP <--- SONICWALL A <--- VPN <--- SONICWALL B <--- USER
If you want them to access it through their WAN connection and that they are connecting to the email server using a public IP address, then you should create two things: One Firewall rule and one NAT rule.
EMAIL SERVER LOCAL PORTS <--- EMAIL SERVER LOCAL IP <--- SONICWALL A PUBLIC PORTS <--- SONICWALL A PUBLIC IP ADDRESS <--- ISP A <--- INTERNET <--- ISP B <--- SONICWALL B <--- USERS
For the firewall rule, you will need to configure it like so:
From Zone: WAN
To Zone: LAN
Service: "The service object/group that contains all the ports through which the users will be accessing your mail server"
Destination: WAN Interface IP
Then you will need to add the following NAT policy:
Original Source: Any
Translated Source: Original
Original Destination: WAN Primary IP
Translated Destination: "The address Object that contains the LAN IP address of your Email Server"
Original Service: "The service object/group that contains all the ports through which the users will be accessing your mail server"
Translated Service: "If the ports of the email server are the same as the one of the original service, select Original, otherwise, select the service object/group that contains all the ports of the local email server."
These are the basic configuration of the rules and you can customize the parameters on the other tabs.
Please note that those rules will also allow users to connect from home or from anywhere else. If you want only the SITE B to access your Email server through their Internet connection, you should change the "Source" of the Firewall rule and the "Original Source" field for the address object that contains the public IP of the SITE B.