The communication limit while sometimes an annoyance is in place to limit spammers. Essentially if you send out too many messages within say 24 hrs you lose the ability to message people for a set time limit. I'm not sure what the message limit is in reference to how many messages you can send or what the time frame is for how long the limit lasts as I have only hit my limit a few times.
Sadly it doesn't take into account things such as friends so there's no way to get around the limit even if you hit it by messaging the same person. I've had people tell me there limit was hit even though that should mean they shouldn't have been able to message me and on rare occasions have re-logged on to find my limit seemingly reset within a short time period, so the limit is hard for me to understand completely. Some people send so many messages that they actually have backup accounts for when their limit is reached. This for example is my secondary account because my original one was hit with a rare anomaly called be silencing glitch that gags a person like a moderator would. When this glitch occurs it pops up as saying your communication limit has been reached when sending messages or posting to the forums yet your limit never resets, but this glitch is extremely rare.