I have this weird #IPv6 problem and it's driving me crazy.
So, hear me out.
I have a #wireguard tunnel set up to a VPS because my ISP doesn't offer IPv6. So all IPv6 traffic is routed over the tunnel. Obviously, this means the MTU should be reduced, so I've configured radvd to broadcast an MTU of 1420.
All fine. IPv6 works. I can visit websites, stream audio and video, ssh all over the place, all over IPv6.
Except for #thunderbird. It'll take ages trying to send an email, then complain the server timed out. It'll be unable to save drafts or sent mail to the imap server, complaining about server timeouts.
So I try sending mail with msmtp from the same machine, all is well, no timeouts whatsoever. It's just thunderbird that's being difficult.
I've been staring at this problem for weeks now, done all sorts of tcpdumps along the path from my computer to my mail server. Double-checked that ICMP6 type 2 is passed along neatly across all firewalls involved.
It's got me stumped.
I'm not expecting anyone reading this to be able to provide a solution, I just wanted to share that it's driving me crazy!