Not possible to specify per-server network proxy

Though possible to specify appropriate system properties for each mail server connection (in servers configuration, "additionalProperties" field), javamail behaviour is not correct.
While they might be effective, those properties will be shared for each connection, meaning that all currently active Sessions will use the last network proxy defined, instead of using their own.

Considering the following Javamail FAQ, fix is only possible using javamail 1.4.5+.
http://www.oracle.com/technetwork/java/faq-135477.html#proxy

So XWiki should use this version of javamail, prior to implement this feature.
(it's possible with previous versions by patching javamail, but we don't want to do that)