What is “spam folder full”?

Spam folder full is a specific quota issue:

What it means:

The junk/spam folder has exceeded its allocation. Does not necessarily affect inbox delivery. May affect handling of messages flagged as spam.

Impact on delivery:

Messages going to inbox: Unaffected. Messages filtered to spam: May bounce or be dropped. Depends on provider implementation.

Why it happens:

User receives lots of spam but never clears it. Separate quota for spam folder. Spam folder not regularly maintained.

Bounce implications:

Relatively rare bounce reason. May see if your mail is being filtered as spam. Indicates your mail is going to junk.

What it signals:

Your mail may be classified as spam. Recipient may not be engaged. List hygiene issue.

A full spam folder suggests you are landing in junk. The real problem is the filtering, not the space.