edits: for clarification of versions, disabling update before reverting to old version, and punctuation.

The release notes for v60.5.2 show changes to the MAPI interface. https://www.thunderbird.net/en-US/thunderbird/60.5.2/releasenotes/

Something in those changes broke the way Sage and/or Peachtree function.

Reverting back to v60.5.0 fixes the issue in Sage Accounting (and Peachtree) (v60.5.1 may work too, but haven't tested it yet)

Download older 32-bit (or 64-bit, if that's what you use) Example (32-bit, Windows, US, v60.5.0) https://ftp.mozilla.org/pub/thunderbird/releases/60.5.0/win32/en-US/

After install of old v60.5.0, reboot was necessary, then Sage(Peachtree) functioned normally.

BEFORE updating, you will need to temporarily disable Thunderbird updates:

Tools menu, Options, Advanced tab, Update tab: set to something OTHER than: Automatically install updates

(If you don't disable updating, then Thunderbird will simply update to the new v60.5.2, and you'll be right back where you started)

Had hoped the MAPI bug(?) introduced in v60.5.2 might be related to the "mapi.blind-send.enabled" parameter, but changing this to either true or false does NOT seem to fix the problem of:

"No Composition Window opening" (in Peachtree - Sage) Accounting when using the email options in that application.

Perhaps other MAPI connected applications are behaving similarly today after updating to v60.5.2?

MAPI is broken in TB 60.5.2. You can follow the issue here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1530820