Outlook prompts with a basic login box when an application tries to send an email via MAPI with the applications exe name (Enter1) in the login title information

 1. Remove all stored credentials under "Manage Credential Manager" > "Windows Credentials" relating to Outlook

2. Delete all Outlook Profiles

3. Delete

   - HKLM\Software\Microsoft\Office (Need to take ownership of some of the folders)
   - HKCU\Software\Microsoft\Office
   - HKCU\Software\Microsoft\Exchange

4. Run a repair of office

5. Reboot device

6. Set up Outlook and make sure modern auth pops

7. Set cache to 6 months

8. Leave it for a few mins

9. Reboot

10. Open Outlook

11. Test
*Outlook needs to be open when sending.

Search Service Tuning on a Remote Desktop Services (RDS) Server GPO Settings

 Recommended GPO Settings

These GPO settings can be updated to turn down the indexing while allowing the Outlook’s instant search feature to function.

• Windows Components/Search
• Allow indexing of encrypted files, Disabled
• Prevent adding UNC locations to index from Control Panel, Enabled
• Prevent adding user-specified locations to the All Locations menu, Enabled
• Prevent automatically adding shared folders to the index, Enabled
• Prevent indexing e-mail attachments, Enabled
• Prevent indexing of certain file types, Enabled
• Prevent indexing public folders, Enabled
• Enable Throttling for online mail indexing, 6 items per minute (maximum is 120 per minute)
• Prevent indexing certain paths:
  file:///C:\*
  outlookexpress://{*}/*
  otfs://{*}/*
  mapi://{*}/*
  mapi15://{*}/*
  ONEINDEX15://{*}/*
  iehistory://{*}/

Copied for Keep sake from

Search Service Tuning on a Remote Desktop Services (RDS) Server - Peters & Associates

A program is trying to send an e-mail message on your behalf warning in Outlook

 If you are using a Remote Desktop host and Outlook, you may come across an issue with 3rd party applications trying to send email.

Outlook will give you an warning saying that "A program is trying to send an e-mail message on your behalf"

The quick fix is to add a reg key (DWARD) setting the ObjectModelGuard to 2 as outlined in the following

https://docs.microsoft.com/en-us/outlook/troubleshoot/security/a-program-is-trying-to-send-an-email-message-on-your-behalf

The location changes based on if you have Office MSI, click to run, and 32 bit or 64 bit installed but its also in the following key

Office\<x.0>\Outlook\Security

x being the version of Outlook installed.

If this does not work you can also try the following keys in the same location

"promptsimplemapisend"=dword:00000002
"promptsimplemapinameresolve"=dword:00000002
"promptsimplemapiopenmessage"=dword:00000002
"promptoomsend"=dword:00000002
"promptoommeetingtaskrequestresponse"=dword:00000002
"promptoomaddressinformationaccess"=dword:00000002
"promptoomsaveas"=dword:00000002
"promptoomformulaaccess"=dword:00000002
"promptoomaddressbookaccess"=dword:00000002
"adminsecuritymode"=dword:00000003

But this was the older way to do it and ObjectModelGuard should be the go to at first.