Exim SMTP not working anymore; connection refused

[mitch2k]

Hi,

at 2am today I started to receive errors from my Nagios machine telling that SMTP was critical on 1 of my VPS servers. Connection is refused
I checked the logs, and it looks like some connections are 'lost' like this:

2013-03-07 20:56:20 SMTP connection from hermes.xxx.be [81.95.xxx.xxx]:39109 lost

Things i've checked:
- Disabled firewall on VPS
- Tested SMTP using external tools

2013-03-07 21:21:47 SMTP connection from mxtb-pws3.mxtoolbox.com [64.20.227.133]:59913 lost (error: Connection reset by peer)

- Added Nagios an mxtoolbox to SMTP whitelist
- Restarted Exim
- Restarted complete server
- Updated OS and software
- Tried to send an e-mail using telnet on my PC. It was accepted and delivered.
- Disabled RBL checks
- Tested connection speed

Anyone has ideas to troubleshoot this problem?

Thanks!