Thanks to the mailing list thread with the guy who wants to remove xz completely, I now know that Dovecot passes --with-lzma to ./configure. Would you mind putting it behind USE=lzma? Support for lzma was deprecated in 2021: > 2.3.14 2021-03-04 Aki Tuomi <aki.tuomi@open-xchange.com> > ... > * Remove XZ/LZMA write support. Read support will be removed in future > release. USE=zlib could also be used to control --with-zlib, but that one would probably need to be defaulted to on to avoid breaking systems using (not deprecated) zlib-compressed maildirs.