@justine I'm new to using it, and I'm "doing it wrong" by using a normal (IMAP/SMTP) email setup instead of a dedicated email server.
Despite that, it has proven to be user-friendly and reliable. It isn't quite as "instant" as other platforms, meaning sometimes it takes on the order of several seconds for messages to be fully delivered and propagated but that's certainly something I can live with. Sharing files, including videos, has worked well on Android and Linux. I haven't tried it with "big" (>10MB) files but even if it didn't work there's other good ways to get that done.
You're actually doing it the right way 😁 the relays have their own code because standard email deployments are configured to get rid of spam at the server level, which is why there are artificial sending limits, overcomplicated configuration, ridiculous restrictions and quotas. If you have a proper IMAP/SMTP server then everything is good. BTW delays are always due to servers, so maybe there's a config on your server that needs to be configured and not on DC's side: postfix for example has an artificial 500ms delay to tell clients a message arrived in the box.