If you want to be able to use your VPOP3 server on devices (e.g. phones & tablets) when they are connected to your office Wifi and also when they are outside the office, you need to be able to configure them so that they can access your VPOP3 server wherever they are.
If your router supports NAT loopback (or NAT reflection), then you can simply use the external IP address, and you'll be able to access it from inside the office as well, however this can be slow with some routers so it is not always a good solution.
However, if your router does not support NAT loopback or you do not want to use it, then an alternative solution is to use two DNS servers.
If you have an in-house DNS server (eg an Active Directory Server), then you can do this easily. If you don't have an internal DNS server then it may be worth getting one (eg SimpleDNS), but you will have to reconfigure all your PCs (or your router's DHCP server) to configure that DNS server as the DNS server to use, which is outside the scope of this article.
Once you have an in-house DNS server, you will usually now have two DNS servers - the in-house one, and one with your domain hosting company.
Now, create a host name on your domain hosting company's DNS server, eg 'vpop3.mycompany.com'. Give this host name the external IP address of your network.
Then, create the same host name on your internal DNS server, but give it the name of the internal IP address of the VPOP3 computer. Depending on your setup, you may not want to create a 'zone' on the internal DNS server for 'mycompany.com', as that will override all *.mycompany.com hostnames. Instead, you can create a zone for 'vpop3.mycompany.com', and set the A DNS record for the zone itself to have the internal IP address of the VPOP3 computer. That way, only that specific name will be overridden by the internal DNS server.
Finally, in all the email clients/devices, use that host name instead of an IP address. Then, depending on where the device is, it will access different DNS servers, which will return different IP addresses.
If you need help with this, we can set it up for you remotely, but since it is not a VPOP3 issue, it will be a chargeable support incident (if you need to purchase DNS server software as well, that will be extra).
Setting up Windows Server DNS Server
If you have a Windows DNS server (e.g. because you have Active Directory, then it is easy to set up a DNS record for your VPOP3 server. One way is to create a DNS zone specifically for your VPOP3 server so it doesn't conflict with any other names.
Go to Administrative Tools -> DNS Manager.
Right-click on the server name and choose New Zone
Select Primary Zone and press Next
If asked, select how you want the zone replicated (eg "To all DNS servers running on domain controllers in this domain")
Select the type of zone as Forward lookup zone
In the Zone name box, put the name you want to use for VPOP3 server, NOT the domain it's in. So, put, for instance, vpop3.mycompany.com
Choose Do not allow dynamic updates
Now, in DNS Manager, select the new zone you have just created, right-click, and choose New Host (A or AAAA)
In the dialog which appears, leave the Name box blank, and enter the IP address of the VPOP3 computer into the IP Address box
This sets the IP address for the zone name (vpop3.mycompany.com in this case) to the specified IP address.