Timeouts for uploading to mailbox.org, ALL SERVICES
For about the last 24 hours or so, I have had two related problems:
1) Sending messages via SMTP that have attachments (3MB ish) eventually time out during the DATA transmission;
2) Copying IMAP messages into another folder (via the APPEND command, which transmits the data) also fails if they are large;
3) Using the web interface at office.mailbox.org, sending a message with a large attachment produces the same behavior.
Over at https://userforum-en.mailbox.org/topic/problems-with-mail-transport-server#comment-1084 a similar problem was reported by another user.
If connected to a VPN, the problem does not exist for me. This implies that it may be specific to certain routes or something.
I eventually ran tcpdump/wireshark across the connection. I have never seen anything quite like it; the two sides remain in communication but get into a TCP retransmit loop. I have fine connectivity to other sites, and cannot duplicate this problem with any other one.
Looking at traceroute (mtr) output, the problematic route to you goes via NTT (eventually Amsterdam then Berlin) before hitting ipberlin.com. The working route goes via be100-1047.fra-5-a9.de.eu and de-cix.ipb.de before hitting the same ipberlin.com router. I cannot say for sure if that's the cause of the problem.
I have replicated the problem on both a desktop and a laptop. I further was able to tether the laptop to my cell phone and that caused the message to transmit fine. If it weren't for another user reporting trouble, I'd suspect I'm all alone in this. I have a very small regional ISP in the USA and it is highly unlikely that another user is using the same one.