Any changes made to mailConfig.xml need to be made while the service is shutdown. Other cases where mailConfig (or domainConfig or userConfig) changes may not take affect.
-An exception occured during loading of the mailConfig (load the _bak)
-(new in 16) If a syntax error is detected, _bak is used.
Bad changes or corruption can cause the above to take place. Otherwise any changes made while it running will just get overwritten with what SM has in memory.