Issue with macOS 10.5.5 / Mail 13.4

[BitingChaos]

This was working with macOS 10.5.4 and previous Mail app. Now I get this:

Incompatible Plug-ins Disabled
Mail has disabled the following plug-ins:
UniversalMailer.mailbundle
Contact the makers of these plug-ins for versions that are compatible with Mail 13.4.

% defaults read /System/Applications/Mail.app/Contents/Info.plist PluginCompatibilityUUID 6EEA38FB-1A0B-469B-BB35-4C2E0EEA9053

Mail has the same UUID, and /Library/Mail/Bundles/UniversalMailer.mailbundle/Contents/Info.plist is using the UUID.

Is there some additional check Mail does with plugins? Any way to get around this?

EDIT:
It's related to System Integrity Protection with macOS 10.15.5!
Booting to recovery, running "csrutil disable", then rebooting back to macOS allows the mail plugin to run again.

[nikzic]

Hi,

I also got the same message about the disabled plugin now that I updated to 15.5.5, however, when I send mails everything looks as if the plugin still works. I.e. my recipients still see the custom font I sent and they do not see any of the weird attachments. I sent the mail to myself from the Mail app, and then opened in in Outlook on Windows and all looks fine - the custom font is still set and there are no those weird attachments. The font was sent both on the UniversalMailer tab in settings (before it was disabled) and the same font is also set on the Fonts & Colors tab. Does this make sense? I do not want to change this on the Fonts & Colors for testing as I am afraid it will stop working. 😀

I am not sure if the plugin is still working somehow for this part or if Mail has been updated so that that the plugin is actually not needed any more.

If anyone has any ideas, please let me know.

[opichals]

Perhaps wwwjfy/GMailinator#1 (comment) can help?

[NemoLi72]

@opichals how to use?

[VitaliyYakob]

Plugin works after updating macOS to 10.15.6!

[BitingChaos]

Plugin works after updating macOS to 10.15.6!

The plugin activates now for me with 10.15.6, but all my messages are being sent without the font being changed.

Is it working for anyone else in 10.15.6?

[VitaliyYakob]

Plugin works after updating macOS to 10.15.6!

The plugin activates now for me with 10.15.6, but all my messages are being sent without the font being changed.

Is it working for anyone else in 10.15.6?

I don't change fonts. My important for me that messages with attachments are sent normally.

[aus-coder]

I usually inject style via Advanced tab and it still works. For example I have this under "Injected Style":

font-family: 'Helvetica', 'Arial', 'Verdana', 'Tahoma'; font-size: 14px; color: rgba(2, 72, 170, 1.0);