So as not to hijack Techguys post, some info on the in box.
Copied from the site help section.
"There is a 100 message limit on the mail folders, so you may have to delete or export some of them from time to time. If you do not keep some space in the mailbox then you will not be able to recieve new messages.
There is a tick box next to each message which allows you to select one or more messages and archive, export, or delete them. If you choose to export you can select which format you want (XML/CSV/Text-File) and save them to your computer."
On my android phone the tick boxes are not visible ,if I want to delete or archive a message I have to use a Computer.
If you archive messages the percentage used value reduces but the number of messages remains the same. So if you have 100 messages and archive 50, your in box will still show as full to other users.
On another subject the time stamp on new conversations really needs to be addressed. In the last few days there have again been a number of new conversations that have gone unanswered for quite a while. I do change the "since last visit" option to "last two days" in order to see missed messages, unfortunately I do not always have the knowledge to help.