lists1001 is still on Buster. Many of the components comprising the Mailman setup are actually as recent as Bullseye (or even more recent/patched), so these need a closer look if we carry local patches etc. But in general from the Mailman perspective we're already quite close to Bullseye:
| Package | Version on lists1001 | Version in Bullseye |
|---|---|---|
| django-mailman3 | 1.3.5-2~bpo10+1 | 1.3.5-2 |
| mailman-hyperkitty | 1.1.0-10~bpo10+1 | 1.1.0-10 |
| mailman-suite | 0+20200530-2~bpo10+1 | 0+20200530-2 |
| mailman3 | 3.3.3-1~bpo10+6 | 3.3.3-1 |
| mailmanclient | 3.3.2-1~bpo10+2 | 3.3.2-1 |
There are various older considerations on use of public IPs covered at (https://phabricator.wikimedia.org/T278495), but it's probably useful to first upgrade lists1001 in place before moving to a new setup.