The survey question is not clear, but confusing ("own server" = docker ?!?),
should it not be:
traditional (non-containerized)
containerized (docker)
This why probably many voted accidentally the opposite way!
Any special install (e.g. into docker) feels like a "nice to have", while the normal/traditional install is a "must have".
Thx for developing MailWizz continuously!
Just in case it is not yet working for you, try with WindScribe VPN and use also their browser extension, so you can make sure there is no time mismatch (use time travel option). Make sure no virus scanner or unknown file hold happens (ie allow to save). Or download it from your server using...
Check each part that is involved, e.g. delivery server, crons, server/firewall, etc.
Test manually in debug mode via CLI.
What specific error message do you get?
Explanations can be found in the ChangeLog in each release zip, in various forum posts, and via experimentation if you create a test campaign incl those tags you want to learn about. In the end, a test camp is needed anyway ;)
Make sure settings/numbers of delivery server, customer, and customer group as well as backend cron settings work together. Also check how much your DS can really take-in, and the latency between your mwz and each DS.