Sempre que preciso migrar mensagens de um servidor para o outro, via protocolo IMAP, uso o Imapsync. Para quem deseja instalar ele em Debian 9, segue abaixo as instruções:

Instalando dependências

apt install -y libauthen-ntlm-perl libcgi-pm-perl libcrypt-openssl-rsa-perl \
libdata-uniqid-perl libencode-imaputf7-perl libfile-copy-recursive-perl \
libfile-tail-perl libio-socket-inet6-perl libio-socket-ssl-perl libio-tee-perl \
libhtml-parser-perl libjson-webtoken-perl libmail-imapclient-perl \
libparse-recdescent-perl libmodule-scandeps-perl libreadonly-perl \
libregexp-common-perl libsys-meminfo-perl libtest-requires-perl cpanminus \
libterm-readkey-perl libtest-mockobject-perl libdist-checkconflicts-perl \
libtest-pod-perl libunicode-string-perl liburi-perl libwww-perl \
libtest-nowarnings-perl libtest-deep-perl libtest-mock-guard-perl \
libtest-warn-perl make libpar-packer-perl libtest-fatal-perl git-core

Baixando o código

git clone https://github.com/imapsync/imapsync.git

Instalando

cd imapsync
make install