The iService.MailPopper Windows service must be running for the system to send and receive email.  So, if your system is not sending or receiving email but there are no error log files, this would be the first thing to check.

When the mail processor is not running all mail will be queued until it is turned back on. As soon as you restart the service all queued mail will be delivered and all mail waiting to be popped in your monitored mailboxes will be processed.

Step 1 - Confirm your mail popper service is running (usually on the iService web server)

Step 2 - Look in the Tools - SMTPOut Monitor to see if messages are being rejected from the mail server

Step 3 - Look in Segment Segments - Mailboxes for errors on mailboxes