I tried to import several thousand emails and failed. It "suggests" that I only send 1000 addresses to begin with. So I uploaded 1000 addresses. I found splitting the list for import to be unreasonable. When I tried to upload more emails, I found out that only 1000 emails can be imported per day.
If the quality of the list is a problem, the bumps and fails might better be identified when the first thousand emails are sent and then stopped if necessary.
But limiting the import is the worst way of ensuring quality.
This hindrance makes it hard to continue trying out this tool.
I have asked support for help and am now waiting to see if there is an improvement. If that happens, I will report here and re-evaluate if necessary.