After upgrading to the latest beta of UniversalMailer (3b19) and Catalina 10.15.3, I found the UniversalMailer plugin was disabled and stopped working. Fortunately, aus-coders posted a code fix for a single .plist file documented below:

1. Quit Mail.app and open up Terminal.app.
2. Inside Terminal.app’s window paste in “sudo pico /Library/Mail/Bundles/UniversalMailer.mailbundle/Contents/Info.plist” and press return to enter your password.
3. Scroll down to the bottom of the document and on the line above “SupportedPluginCompatibilityUUIDs“, paste in the following code:

Supported10.15PluginCompatibilityUUIDs

# UUIDs for versions from 10.12 to 99.99.99
# For mail version 12.0 (3445.100.39) on OS X Version 10.14.1 (build 18B45d)
A4343FAF-AE18-40D0-8A16-DFAE481AF9C1
# For mail version 13.0 (3594.4.2) on OS X Version 10.15 (build 19A558d)
6EEA38FB-1A0B-469B-BB35-4C2E0EEA9053

4. Press the “control” and “x” keys and then press “y” to save.
5. Launch Mail.app reactivate the UniversalMailer plug-in.

In the future, it may be necessary to use this fix to patch the .plist file for future versions of Catalina (and beyond). The code in step 3 can be updated for future versions of Mail.app, as long as you get the updated UUID, which is the long string of numbers and letters separated by dashes. To get Mail.app’s updated UUID, use the following:

1. Open Terminal.app and paste in “sudo pico /System/Applications/Mail.app/Contents/Info.plist”.
2. Press “control” and “W” and enter “UUID”. Copy the line beneath “

PluginCompatibilityUUID

“, which looks like this for Catalina 10.5.3:

6EEA38FB-1A0B-469B-BB35-4C2E0EEA9053

3. Paste this code following steps 1 through 5, up above.