I checked our SMTP logs in a couple of live environments we have in-house and didn't see any exponential growth in ours so my bet is there is likely something happening simultaneously and the timing is likely coincidence. Very curious to see what the bulk of the extra log traffic is! 

I have an email blast scheduler setup to send about 8000 messages at 5am.  Most systems I run, I have upgraded to queue the messages to the DB server, then a separate script picks up a few messages from the DB, and sends them on through smartermail every minute.  This helps to trickle out the messages and not over load the receiving servers.  On this particular system it was not upgraded to queue the messages, and instead just forced all 8000 messages through smartermail as fast as possible.  When I looked at the Logs,  90% of the lines in the SMTP logs looked like this ( have changed the ip, domain and Alias for privacy reasons)

05:06:53.552 [1.2.3.4][62213893] senderEmail(2): news@mybusiness.com parsed using: "My Businesss" "><news@mybusiness.com>

From 5:00 AM through 6:45AM there are about 30 million lines that all look like that.

I have since updated the code in my application to queue the messages, before trickling them on to smartermail.

Hoping that resolves the issue, but still never had this issue before upgrading to the latest version of smartermail.

Interesting finds! That definitely sounds like our source of trouble there. I think we added some additional logging details recently but I'm not aware of anything in this area specifically that would impact the log sizes. Is it possible the first attempts failed and so it retried, adding to the amount of log entries for this session? If you can submit a ticket on this one I'd be happy to take a look and make sure there isn't anything deeper going on for you.