The attached mbox file causes movemail to loop endlessly between _fromrd_encoder (which returns mu_filter_moreoutput) and filter_read, which apparently does not invoke _fromrd_encoder with a larger output buffer.
This should reproduce it: gunzip hang.mbox.gz movemail hang.mbox output This happens with Debian's mailutils 1:3.15-4 and the current code in Git (at commit f3913a352c92c3b3b4df943037c0b392fda9463b).
hang.mbox.gz
Description: application/gzip
