A user with a Macbook running OS X El Capitan connected to the Turris Omnia router over wifi.
They could browse the web but when they try to use Thunderbird (v45) to connect to their IMAP server (TLS on port 993), the TLS handshake doesn’t complete.
I manually set the MTU on the Macbook down to 1280 in case it was an MTU issue.
I ran a packet capture on the router and observed retransmissions and Dup ACK messages.
A Linux laptop running Thunderbird is fine on the same wifi.
Has anybody seen similar problems with Mac OS X and Turris Omnia / OpenWRT?