“Unable to sign or encrypt message. Certificate not installed”

Since there are 2 places you can have the digital signature for outgoing emails enabled on the Email app, then you should ensure this feature isn’t turned on in both of them.

i. Turn off digital signature for all outgoing emails.

  1. Launch the stock Email app.
  2. Tap the Menu button and then select Settings.
  3. Tap the account which you like to turn off the digital signature feature.
    email settings
  4. Next, scroll to the bottom, and then tap More settings.
    email settings_1
  5. Again, scroll to the bottom and then select Security options.
  6. Untick Sign if it’s selected.
    email security option

ii. Turn off digital signature for individual email.

  1. On your compose email screen, tap the Menu button.
  2. Select Security options.
  3. Untick Sign if it’s selected.
    outgoing email security option
Note: Some users reported that when someone sends them a signed email, on reply it’ll have the Sign option checked automatically. In that case, you’ll need uncheck it manually. Your phone may or may not have the same behavior.