Emails give 'Excessive errors processing emails' with 'Invalid header value detected'. Specific client emails fail; forwarding them through webmail fixes the issue. Happens with Exchange and Google Workspace senders.

Set Log Level to DEBUG in Admin Panel > Settings to surface more detail. In class.mailfetch.php add error_log($t->getMessage()) around line 155 to expose the raw error. The root cause is often a malformed or unusual mail header (e.g. X-Ham-Report) that the older PEAR library cannot parse. A community workaround is to wrap processMessage() in a try/catch that logs and skips bad emails rather than halting the fetcher. Official fix is targeted for v2.0 which will use modern mail libraries.