all/modules/htmlmail/htmlmail.mail.inc on line 188

This behaviour is caused because mailsystem version 7.x-3.x does not load mailsystem.html_to_text.inc automatically on each and every page request using hook_init. Attached is a patch for htmlmail taking this change into account by selecting and loading the appropriate include file only if necessary.

Actually this is duplicate of #1692966: Incompatible with latest MailSystem code but I favour this solution.

Thank you sooooo much for this solution!!!
I was trying for days to get my email working from my drupal site.
This patch was the answer to my problem.

I reviewed and tested this and it worked great!

+1 - rolled patch #1 against HTML Mail 7.x-2.65 (can't see a DEV release)

Can we roll this into the latest release so we don't have to keep patching the module?

this is ugly bug, tested against current, please commit it and make a new version ASAP

Take care of this patch as "enhancedtextmail" is a submodule of mailsystem proposed in a patch but without any evolution for a year.
More, this patch tries to load the file mailsystem.html_to_text.inc which does not exist.
I suggest you to rely on #1692966: Incompatible with latest MailSystem code for the moment.

Marking as a duplicate of #1692966: Incompatible with latest MailSystem code.