I'm forwarding Debian bug 635142, http://bugs.debian.org/635142 , reported against svn revision 43:
I don't know in which version it started to happen, but checkgmail is now ignoring https_proxy environmental variable.
I check this bug using wireshark.
I tried a lot of combinations, but essentially:
$ https_proxy=http://x.x.x.x:x checkgmail
$ HTTPS_PROXY=http://x.x.x.x:x checkgmail
It worked for me in the past, but suddenly it has stopped.