One step further in my quest to create a replacement mail server.

I now have my old mail server (2.3.19.1, macOS + MacPorts) and my new (2.3.20, Alpine Linux, Docker, apk package). When I turn on replication it works, but, after a while I see:

Jan 06 00:50:31 replicator: Panic: data stack: Out of memory when allocating 268435496 bytes
Jan 06 00:50:32 replicator: Fatal: master: service(replicator): child 133 killed with signal 6 (core dumped)
Jan 06 00:50:32 lmtp(pid 195 user sysbh): Warning: replication(sysbh): Sync failure: 
Jan 06 00:50:32 lmtp(pid 195 user sysbh): Warning: replication(sysbh): Remote sent invalid input: -

I've removed synchronous operation for now (found a message on the net suggesting that) but is this known and what does it mean?