Howto increase user mail quota who is already registered:
Open Domino Admin
Goto FILES / MAIL
on your right , Right click on user and select QUOTA
As shown in the image below . . .
Now click on User and set its database limit as per requirements.
For more info,
http://www.ibm.com/developerworks/lotus/library/domino-mail-quotas/
Regard’s
Syed Jahanzaib
