When sending e-mails after 2.2.0, where multipart e-mails were introduced, the final e-mail is usually sent out with a text/html content type, and the plain text part and the boundary headers are output.
Tried 3 different SMTP plugins
fluent smtp: won't sent (error code below)
WP Mail SMTP: sends - but includes the error text below
Post SMTP Mailer: sends, not not plain text. also includes some of the broken code below.
So far SendGrid is a confirmed provider where things break.
Describe the bug
When sending e-mails after 2.2.0, where multipart e-mails were introduced, the final e-mail is usually sent out with a
text/htmlcontent type, and the plain text part and the boundary headers are output.So far SendGrid is a confirmed provider where things break.
More context: p1707231412995879-slack-C6GGX896G