Cant connect to addon domains email

Hello Everyone, I noticed a problem today with one of my Accounts and i was wondering if it should happen. I have an account on my cpanel named for example account1, i have put on that account 3 addon domains. I created emails for all of the accounts and i encountered this problem. The main domain and the addon domains have access through the webmail (on the server) but when i tried to connect to the addon domains email addresses from an email client on my pc there was a password error, and the main domain's email could log in just fine. I tried it with other accounts on my server and it does the same thing. (I am root administrator at the server, It's a VPS server, and i tried two email clients on 2 different PC's thunderbird and operamail )