I assume this is a bug. I found a way around it that I thought I would post in case someone else ran into this issue. I had a customer who was getting the mailbox quota exceeded message and could not send/receive email. I went in and changed the default mailbox size for the domain, which then showed that the users had plenty of additional space in the mailbox accounts. However, they were still receiving the mailbox exceeded quota error and couldn't send/receive email. Plesk was showing they had plenty of space and were well below their quota. Their mailboxes were all set to use the default mailbox size for the domain. However, when I went in to one of their accounts via webmail, Horde was saying their quota was now 0MB. When I went into the specific mailbox and changed it from using the default mailbox size to a specific number slightly less than that, it changed the mailbox quota size like it should have in the first place. So the users were then able to send / receive email.