The quota of IMAP mailboxes is 500MB, and cannot be increased. In order to make space for new incoming messages, you will have to delete emails that are in the mailbox. If you would like to save these messages, you can utilize a local client like Outlook or Thunderbird, and copy the messages to a local folder outside of the RPI mailbox before deleting them from the server.
Messages that are put into the Trash folder in IMAP still count against the quota. To effectively delete messages when you are over quota, you can empty the Trash folder:
1. Log into Roundcube (Webmail) https://webmail.rpi.edu
2. Click the Trash folder to select it.
3. On the lower left corner, click the gear (Folder actions) button. Select "Empty" from the options given.
4. Click "OK" in the pop-up window to confirm deletion of all messages in the Trash folder.
Your quota usage is listed in the email page in the lower left corner.
Mailbox so full you can't move items into Trash?
On the upper right corner, click the gear (Settings) button.
On this page there are three sections. Under Settings, select Preferences. Under Section, select "Server settings".
In the right section, check the box next to "If moving messages to Trash fails, delete them."
Click the Save button to confirm your changes.
Note that this means any items that are now deleted will be permanently removed.