queued messages should be able to use `\n` instead of `\r\n`

[FelixSchwarz]

Since commit a045882c868be93ddf160aeeb11f02b45da49657 the queued emails on disk are assumed to use CRLF so the emails can be sent as-is over SMTP. This was the quickest way to implement the CRLF change but I dislike that assumption.

At some point I would like to revisit that assumption.