Right after I get through fighting with with my VAT submission software I'm going to write a kplex/marinetraffic FAQ...I learned a lot playing with this myself.
Here's my guess as to what's happening. You dropped a connection to your router / were issued with a new dhcp address. Check your messages file an hour before the offline mail and my guess is you'll see something about the network interface going down then up. kplex tried to write a udp packet, got an error and the marinetraffic interface shut down. There is no "persist" option for udp interfaces because hey, it's connectionless. But I forgot about interfaces up-downing and that pesky dhcp. I have it on the "to do" list but it's non-trivial and I have a lot on at the moment.
Options to stop it happening again?
1. Try using a fixed IP address on your outgoing network interface
2. use a dhcpcd hook to restart kplex when dhcpcd gets a new address (I will hopefully document how to do that sometime in the coming week)
3. use tcp with "persist=fromstart"
As discussed in another recent thread, if you have particularly low volumes of data and want to stay "online", be aware that:
- marinetraffic regularly (every hour in my experimentation but I understand they vary this) restart their servers which will disconnect a tcp connection
- kplex will only notice an outbound-only connection has been dropped when it tries to write data and fails
...so your choices are:
1. Remain offline until there's something else to send
2. Make the connection to marinetraffic bi-directional. kplex notices the connection closing immediately for read interfaces. You might want to add "ifilter=-all" to that interface, although marinetraffic won't send you anything
3. Use the heartbeat option implemented in the develop branch to send a valid-but-meaningless nmea sentence to marinetraffic once every so often (e.g. 10 mins) which will make kplex discover a dropped connection