send-email: make message-id generation a bit more robust
Earlier code took Unix time and appended a few random digits. If you are firing off many messages within a second, you could issue the same id to different messages, which is a no-no. If you send out 31 messages within a single second, with random integer taken out of rand(4200), you have about 10% chance of producing the same message ID.
This fixes the problem by uses a prefix string which is constant-per-invocation (time and pid), with a serial number for each message generated by the process appended at the end.