Detail of the message that has popped up ...
"Your mailbox storage will be full in 18 days"
"Upgrade for more storage or free up space. If your storage is full after 15 May, you won't be able to send or receive emails."
"Your mailbox storage will be full in 18 days"
"Upgrade for more storage or free up space. If your storage is full after 15 May, you won't be able to send or receive emails."
A hybrid of this would be to move folders that you don't really use any more and just have for archival purposes. Whatever it takes to free up space.
The other option of course is to pay them for more storage.