Issue:A receive connector was set-up on ExchangeSrv#1 to allow relays from 3rd party of line of business applications. ExchangeSrv#1 had plenty of space, not low on resources and was not in a back pressure condition. A 3rd party system was in place and submitting emails using Telnet.
The majority of emails being submitted were being rejected with the error "453 4.3.1 Insufficient system resources". ExchangeSrv#2, did not have the same receive connector configured and was not accepting emails from the 3rd party system. It did however have little of no disk space left on the C:\ drive which also hosted the transport queue file.
In the event viewer ExchangeSrv#2 was in back pressure. It was discovered that multiple IIS logs files were consuming space on the C:\ drive of ExchangeSrv#2. A process was put in place to truncate the IIS logs. After clearing space on the C:\ drive of ExchangeSrv#2, we were then able to submit emails again to ExchangeSrv#1.
Reason:So why was the low disk space and back pressure issue on ExchangeSrv#2 the cause of the insufficient system resources error on emails submitted ExchangeSrv#1. In short, it was because of Safety Net in the Transport Role.
By default emails submitted and processed on an Exchange server are kept in the mail queue for a set period of time. Copied of these emails are also kept in the mail queue on Exchange server in the same local Active Directory site.
Because there was no space left on ExchangeSrv#2, Safety Net features couldn't be achieved and therefore the emails were rejected.