Problem with mail download

Problem with mail download

I have two mailboxes on Zoho and three mailboxes on other servers which I download with my Outlook (2007) via pop/smtp access. Last week the problems started, and only my Zoho mailboxes are effected:
- for one of my Zoho mailbox Outlook reports "errors", no downloads possible
- from my other Zoho mailbox some mails arrived with a delay of two or three days
- a lot of them (google alerts) appeared in my junk mail folder
- a lot of them appeared without sender and subject
- on my account on the Zoho website, the mails are there, I can access and forward them to my other mailboxes and receive them without delays
- currently my Outlook is downloading some 600 mails / 250 MB, but 1) there are only some 50 unread messages in my Zoho mailboxes and 2) the mails appearing in my inbox is much lower than the number of allegedly downloaded mails Outlook is stating in the display in the bar below

My AV programme Bitdefender reported a bug last week, shortly after the problem started, but works fine since.

Any idea about the cause? What I can do?