nested messages have a blank line preceeding Content-Type and Content-Transfer-Encoding

[samwyse]

This may be an issue with at least two different mail agents, but I thought I'd ask here first. I subscribe to users-digest.tomcat.apache.org, which I read using Outlook 2019. Normally, I receive a message with one attachment, which I open to see one or more attachments and a list of their subject lines. Most of the time I can open the attachments without any issues, but occasionally I get a message body that looks like this:

Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: base64

[hundreds of kilobytes of base64 encoded data]
--------------IPvxRweP6IpEOFPMjFv6NuMK--

These messages are from one specific user. I would contact them, but I just now noticed that I also get message bodies from someone else that look like this:

Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

[human readable data]
--------------Sur02Z02erq39KBVEM60HC8K--

The latter message looks normal in the mail archive.