A MX lookup for your domain gives this result:
- Name=imagineandthink.zapto.org
Type=MX, Class=1, TTL=60 (1 Minute), RDLENGTH=4
Preference=10, Mail Exchange=imagineandthink.zapto.org
Additional Records Section:
- Name=imagineandthink.zapto.org
Type=A, Class=1, TTL=60 (1 Minute), RDLENGTH=4
IP Address=121.215.144.174
However, I'm not able to connect to 121.215.144.174 (no ping response, no reply on port 25, no reply on port 110). In other words, the MX record is formally correct, but there is no server to be found at that IP address.
So: Make sure the MX IP is right, make sure the required ports aren't blocked by a firewall, make sure SMTP and POP3 ports are forwarded to the local server IP from your Internet router/modem. Furthermore, to connect locally to the server on port 110 use the LAN IP address, not the public IP.
This is the networking part. Besides that there might be things to check in Mercury configuration, but we can come back to that when the network issues are resolved.
Needless to say it's much easier to set things up with a fixed IP if that option is available.
/Rolf