I keep getting errors "malformed MIME header line" or "malformed MIME header key", and I know that aerc uses go-message to fetch and parse IMAP messages.
So, I created a github issue in go-message repository with examples and detailed explanation: https://github.com/emersion/go-message/issues/144
But it's maintainer said:
Seems like an aerc bug. It tries to parse message bodies as headers.
UPD: After discovering this fork, I tried again with aerc-0.6.0 -- problem persists
Before I tried v0.5.2 and 0b19b5e70e408bbaac5555b0b61a9451189406f8 commit (latest from old repo)
I cannot reproduce the issue with the example message you provided. Did you check with the current master?