What does "Convert ... upon sending" mean?

[Paul Gilmartin]

On 2019-04-21, at 15:29:05, Kevin J. McCarthy wrote:
>    ...
> What appears in your Sent folder is what Mutt put on the wire, so if that's correct then most likely this is not a Mutt issue.  Perhaps you should try again with 'iso-5589-5' and see if it helps.
> 
The problem does not occur with Yahoo's SMTP server and either Yahoo's or AOL's IMAP server.

Blame AOL's SMTP.

Thanks,
gil