Mail is blocked, additional storage space didn't help

Mail is blocked, additional storage space didn't help

Dear team,

our mail account is blocked due to the aailable storage space (5 MB) being full. I have just bought more storage space - but it is not available to the mail account. WHAT CAN I DO??? I would love to buy more mail storage space, but apparantly cannot find the correct way to do so.
Please advise!

Our plan is "Professional edition (1 User)"
Mail storage is 5 GB and I seem unable to upgrade it.
Under Setup  > Data Administration > Storage it says there is 6301.36 MB total free space .

This is urgent since we cannot send or receive mails. Please help.
Thanks
Sylke Sedelies