In Outlook 365 the From field is blank. You are also unable to change signature assignments in the Signatures dialog after upgrade from version 2002 (12527.20194) to version 2003 (12624.20176).
This was fixed in an update released late on March 13 2020. Check for updates and restart Outlook if you are still affected.
This issue only affects users with both Exchange accounts (including Outlook.com) and POP or IMAP accounts in their profile.
Microsoft article on it: Cannot select From account in email with Outlook Desktop
This doesn't affect the features from working. The correct accounts are used if you reply and the account that owns the current folder is used for new messages. Signatures will be added, provided they were assigned in an earlier build.
Microsoft is expected to release a fix for this within the next couple of weeks, but until then you have options if you can't wait for the update:
- Remove the Internet accounts from the profile.
- Rollback to version 2002.
- Split the accounts into 2 profiles, one for Exchange and one for Internet accounts.
- If you just need to change the signature, you can edit the registry.
Rollback to 2002
If anyone needs to rollback, it is easy enough. Don't forget to turn off automatic updates for a few weeks or the current build will be installed again.
Type cmd on the start menu, right click on the Command Prompt, choose Run as Administrator then copy and paste the line in the command prompt. This will install version 2002:
"C:\Program Files\Common Files\microsoft shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.12527.20194
If you want to use a different build, get the build # from Release notes for Monthly Channel releases in 2019 and update the command line.
Signatures are stored in the registry, in the account subkeys (000000nn) in your profile. You'll need to browse the subkeys to find the correct one for your account. Edit the signature keys or add them, with the signature name as the value.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Profiles\Outlook\9375CFF0413111d3B88A00104B2A6676 REG_SZ: New Signature REG_SZ: Reply-Forward Signature
Split the accounts into two profiles
Splitting the accounts into two separate profiles easy enough: copy the profile then remove the Internet accounts from one copy. (This is my least favorite option.)
- In Outlook, go to File > Account Settings > Manage Profiles
- Click Show Profiles
- Select the Profile and click Copy
- Select a profile and click Properties then remove Internet accounts from the profile.
- Repeat for the second profile.