The ^MONMGR utility just send mails based on the severity of the messages in the cconsole.log. In order to find the severity in the cconsole.log you can look at the number after the date and process id. 

11/14/16-00:39:09:735 (74654) 2 [SYSTEM MONITOR] WriteDaemon Alert: Write Daemon still on pass 1112
11/14/16-00:40:40:865 (74654) 0 [SYSTEM MONITOR] WriteDaemon Alert: Write Daemon completed a pass.

In that case we can see severity 2 for the first message and 0 for the second one (The OK one). As the default severity level in ^MONMGR is 2, it will only send mails when it's 2 or higher.  Note that you can change this level from the MONMGR menu, but lowering to 0 or 1 will give you lots of messages which probably you don't want. 

Anyway, after getting any 2 level message, the recommendation is to review the logs and find what caused the problem. If is a puntual error and was solved by itself (like the one you described) you should not worry. If you keep getting such messages my recommendation is to investigate and contact with Intersystems support if you need help. 

Just a note of attention: 

Many people find that the system is taking a lot of space because they have never purge any message. In that case, do not purge all the messages at one! Purging messages will create journal entries, so if you do a very big purge you can expect a lot of journal growth and you have to take it into account. 

My recommendation would be to purge in chunks, controlling how much journal files increase and how much space left you have.