Mail garbled and headings switched

I have an iMac (about 2 years old) running 10.13.3

My mail has started becoming garbled - not all of it. Here's an example.

--94eb2c1af38207e4c905664a2075

Content-Type: multipart/ALTERNATIVE;

boundary=94eb2c1af38207e4bc05664a2073

--94eb2c1af38207e4bc05664a2073

Content-Transfer-Encoding: 7BIT

Content-Type: TEXT/PLAIN;

charset=UTF-8

I'm also suffering from headers being switched. i.e. the header will refer to a different message.

I've rebuilt the mailbox several times and I've restored the system fonts.

I have no problems with the same messages on my iPad and iPhone, which both display perfectly.

Any thoughts?