The best answer can be taken from Gmail support. Yes, it's possible that they block your Mumara server's IP after seeing so many connection requests to several accounts from the same IP and sending unsolicited emails. And when you connect to a remote server e.g. Gmail, the default route IP is selected from the server even if you have a lot of additional IPs in the server. Sockets are always opened using the default IP in the routing table. So either you'll need a custom scripting that will keep changing the default IP on every connection or after some time, or integrate a VPN at the server level. But again, if you manage to change default IPs with the additional IPs, you'll still be banned as the scope is limited and every IP will be repeated after a few seconds.