[Exim] bug: another LHLO problem

If you do recipient verification with callouts, Exim sends "HELO" even
over LMTP. The fix shouldn't be too difficult.

Exim's rejectlog says something like:

rejected RCPT <mze1>: response to "HELO mail.dcs.qmul.ac.uk" from vicar
[138.37.88.163] was 500 5.5.2 Syntax error

Cheers,

Matt