Hello Maxim,
Usually is normal setup of EOip tunnels though transport ipsec (transparent lan). And from security prospective the most bigger threat is coming from inside. Outside intrusion possible, but it match more complicated.
I confirm that plain 143 proxy working good. I just wonder about this message.

2013/12/05 00:05:40 [error] 20003#0: *1 auth http server 127.0.0.1:80 did not send server or port while in http auth state, client: 10.12.130.102, server: 0.0.0.0:993, login: "testuser1"