I like to use rspamd to process my messages including outbound email. It generates this symbol for messages generated with ALPS MISSING_MIME_VERSION(2) Adding header like MIME-Version: 1.0 might be very simple task and it may greatly affect spam score of outgoing emails.
go-message 0.12.0 should take care of it. We just need to
go get -u ./... && go mod tidy
.