Nylas accounts connected to email servers hosted by Redtail / Zimbra (redtailtechnology.com) have difficulties sending, synchronizing, or staying connected. Get 504 errors when sending.
Redtail / Zimbra throttle connections from Nylas as they are unable to support our connection throughput.
If it's a failed send, backoff and retry.
For synchronization, there is no solution. You will see slower synchronization speeds and it will take longer for items to show up in Nylas.