USER MODE m
Usage: UMODE +|-m
Usage: MODE <nick> +|-m
m - Message block
This will protect you from getting direct messages from other users. This is useful if you are away and do not want people trying to communicate with you or if you do not want to be bothered outside of channels. It also is a good mode to set for children.
While it is best to either supervise a child or educate them thoroughly and only allow them to chat unsupervised when their guardians feel they are ready, removing private messages provides some additional protection. Then if they remain in well-supervised channels the operators may make sure that the conversation is acceptable.
/mode jimmers +m
You can define a number of exceptions for this block, see: UMODE +B, UMODE +C, UMODE +J, UMODE +M, UMODE +R, UMODE +W
USER MODE M
Usage: UMODE +|-M
Usage: MODE <nick> +|-M
M - Registered users on channel exception for +m
This mode is only effective if you have UMODE +m set, it allows users to message you if they have a registered nickname and are in a channel with you.
By using +m and +M, you can make sure only registered users you can see in a channel are allowed to message you.
/mode jimmers +M
See also: UMODE +R and UMODE +C
Note: The difference between +mM and +mCR is, that +M only allows users that are registered AND in the channel, while +CR allows users that are registered OR in a channel with you.