After starting a KDE session, kwallet asks for the password for the password wallet, as soon as it is required. 
The first required passwort from kmail for sending or receiving emails is always required to be re-entered, all other mail passwords are remembered by kwallet, except the first one. 

Example:

1. Startup Kde
2. Enter Wallet password for kwallet
3. I have several pop & smtp accounts: a, b, c
4. If account a checks for mail (pop) or sents mail (smtp) first, i have to enter the password for account a as kwallet mysteriously does not remember, checking b or c will then not require to enter a password as kwallet remembers these passwords
5. If account b is the first kmail component to ask for a password, this password is not remembered, but accound a and c are remembered.

repeatable:
everytime

kde version:
4.8.4.

I have the same issue with kubuntu packages 12.04 on a clean install on a different machine.

I ll be happy to investigate when instructed