[ALUG] Claws mailbox over quota?

Following a query from Mick, I decided to try importing all my mail from Thunderbird into Claws (v3.8.0) and it's all been going well until I tried to import a mailbox from 2009. The messages in the import folder total 399 and yet there are only 350 in the imported folder. I clicked on the red exclamation mark at the bottom of the Claws window and it tells me '*** IMAP4< Alert: Mailbox is at 99% of quota' but despite searching everywhere I can, I can't find any mention of quotas. Can anybody help please?