On 6/22/22 02:18, tim Rowledge wrote:
> The non-broken mail
> a) doesn't break within a tag
> b) has 4k long lines instead of 998 char lines.

MIME has a line length limit. From 
https://datatracker.ietf.org/doc/html/rfc2822#section-2.1.1:

    There are two limits that this standard places on the number of
    characters in a line. Each line of characters MUST be no more than
    998 characters, and SHOULD be no more than 78 characters, excluding
    the CRLF.

It looks like maybe WAHtmlCanvasBuilder is producing a single long line 
of HTML (commendable!) but them that MIMEDocument isn't honouring the 
length limit.

Yep:

    (MailMessage new body: (MIMEDocument contentType: 'text/html' 
content: (String new: 5000))) text lines collect: [:line | line size]
    --> #(37 23 0 5000)

Setting a content-transfer-encoding header fixes the line-length 
problem. I expect you'll not have deliverability problems with base64 
encoding, but you could try that or quoted-printable:

    (MailMessage new body: ((MIMEDocument contentType: 'text/html' 
content: (String new: 5000)) setField: 'content-transfer-encoding' 
toString: 'base64')) text lines collect: [:line | line size]
    --> #(37 23 33 0 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 
72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 
72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 
72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 72 
72 72 72 44)

Cheers,
   Tony

(Just an aside: you have two "Content-type" headers in the sample you 
posted, and while email header names are expected to be processed 
case-insensitively, I am superstitious about these things and would have 
it with a capital "t", "Content-Type"... well, I'd go look at the spec 
and do what it suggests in there...)