Apple mail frequent password confirmation for apple mail server

Hi there,

we have a mac mini server with Mountain Lion. Apple mail frequently asks for password confirmation (IMAP accounts), not permanently though. It will not accept the right password and asks again several times before it works. It takes about one hour and then the password is required again. At first it started on may collegues iMacs, I had no problems on both of my Macbook Pros so far until now. I already deleted the passwords saved in keychain and saved them again, password is correct. It happens wether I am in the local network or in any other. I searched forums but found only hints for peolpe using other email providers such as Yahoo. Can anybody help me with this problem? I do have some knowledge about these things but pretty basic when it comes to settings for mail accounts, so please try to explain as you would explain your grandparents ;-) Thanks!