Apple released macOS Catalina (10.15) last month. Mailplane 4.1.4 runs well on it.
You can read a bit about the new features of macOS Catalina here.
Attachment downloads didn’t work in Mailplane 3 off the bat but we did release an update today. Please do check for updates via “Mailplane 3 > Check for Updates…” or grab the latest version from our download page.
Please keep in mind that we don’t actively develop Mailplane 3 anymore and therefore have no plans for further updates. Give Mailplane 4 a try if you want to stay current.