Private messaging and signature edition/creation functions are not available to accounts with less than 3 posts count.
It is done to:
1. prevent bot spam in private messages.
2. eliminate non-human registered users to join the forum for the purpose of building advertisement link in their signatures.
Also note that your mail box capacity will increase as your forum rank increases. Your forum rank is directly related to the number of posts you have:
Forum Rank Required Posts Mailbox Capacity
Newbie 0 5
Payon Villager 3 10
Morroc Trader 10 35
Prontera Adventurer 50 75
Al De Baran Alchemist 100 100
Lighthalzen Researcher 200 150
Yuno Scholar 500 200
New World Alliance 1000 250