reading the exim log

I'm trying to understand my exim log. I have enabled extended logging with the exim config and I'm getting a lot of the following, perhaps someone could shed some light on what it means, TIA.
2005-01-27 02:04:54 SMTP connection from [212.163.168.xx]:1656 I=[69.90.172.xx]:25 (TCP/IP connection count = 1)
2005-01-27 02:04:56 SMTP connection from (spica.mail.cambra.xx) [212.163.168.xx]:1656 I=[69.90.172.xx]:25 closed by QUIT
I've added the xx in the ip numbers .
This will appear 4 or 5 times in a row, obviously [212.163.168.xx]:1656 is the person connecting to [69.90.172.xx]:25 (which is my server) but how and why are they connecting and what are they doing? Why are they being closed?

 

 

 

 

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