Thast just a connect, can be a wrongly configured monitoring tool from a differnt server os a postfix instance that verifies the existance of your server before it delivers a email. so nothing to worry about.
Thats not the setup that ISPcconfig uses. So your server must break if you apply anthing from this page to an ispconfig system.
The only setting I applied was "content_filter" on the main.cf... as you may read on the doc link. But I check all the other settings on the doc link and they matched my settings: /etc/amavis/conf.d/15-content_filter_mode and /etc/postfix/master.cf althoght I don't remember the original settings for the master.cf "pickup" -o content_filter= -o receive_override_options=no_header_body_checks
and thats the reason for your problem. Either you or another person that administrates this server must have altered the transport from "amavis" to "smtp-amavis" in master.cf, as ispconfig has not done that and the doc link you posted described a differnt setup then the one that ispconfig uses, so do not follow the instructions from the wrong setup of the ubuntu site. To finish this thraed: To fix your setup, rename the transprt back to amavis in master.cf and rename the content filter back in main.cf and restart postfix.