Action: Send Mail in Rich Forms and Action Links

Send Mail action allows dynamically sending emails from your SharePoint site to any email address.  

Example of Send Mail action configuration* 

* Configurations are available for actions that use the Microsoft Graph Send Mail API. 

Note:  In order to use the Send Mail action in Rich Forms and Action Links you have to approve Microsoft Graph 'Mail.Send' delegated permission for the SharePoint Online Web Client Extensibility. This should be done once per tenant by a tenant admin.  
To approve Microsoft Graph 'Mail.Send' delegated permission please run the PowerShell script below. Alternatively, you can download the attached script file (below) and run it using PowerShell. 

$msGraphAppId = "00000003-0000-0000-c000-000000000000"
$scopeMailSend = "Mail.Send"

$spOnlineWebClientExtensibilityAppId = "08e18876-6177-487e-b8b5-cf950c1e598c" # SharePoint Online Web Client Extensibility
$resourceGrant = $null

# Prompt to install the required modules if not yet installed
if ($null -eq (Get-Module -ListAvailable -Name Microsoft.Graph.Applications) -or $null -eq (Get-Module -ListAvailable -Name Microsoft.Graph.Identity.SignIns)) {
  $response = Read-Host -Prompt "Running this script requires Microsoft.Graph modules that are not yet installed. Install now? (Y/N)"
  if ($response -eq "Y") {
    if ($null -eq (Get-Module -ListAvailable -Name Microsoft.Graph.Applications)) {
      Install-Module -Name Microsoft.Graph.Applications -Scope CurrentUser -Force -AllowClobber
    }
    if ($null -eq (Get-Module -ListAvailable -Name Microsoft.Graph.Identity.SignIns)) {
      Install-Module -Name Microsoft.Graph.Identity.SignIns -Scope CurrentUser -Force -AllowClobber
    }
  }
  else {
    Write-Host "The script cannot continue without the Microsoft.Graph modules. Exiting." -ForegroundColor Red
    exit
  }
}

Connect-MgGraph -Scopes "Application.ReadWrite.All", "Directory.ReadWrite.All" -NoWelcome

try {
    # Get the SPFx Service Principal
    $spfx = Get-MgServicePrincipal -Filter "appid eq '$spOnlineWebClientExtensibilityAppId'" -ErrorAction Stop
    # Get the endpoint service princpal (required to identify the object ID)
    $resource = Get-MgServicePrincipal -Filter "appid eq '$msGraphAppId'" -ErrorAction Stop

      # Get the scopes granted for the endpoint
      $spfxGrants = Get-MgServicePrincipalOauth2PermissionGrant -ServicePrincipalId $spfx.Id -ErrorAction Stop
      foreach ($spfxGrant in $spfxGrants) {
        if ($spfxGrant.ResourceId -eq $resource.Id) {
          $resourceGrant = $spfxGrant
          break
        }
      }
      # If some scopes have already been granted for the endpoint, we check if the scope we are about to add already exists there
      if ($null -ne $resourceGrant) {
        if ($resourceGrant.Scope | Select-String $scopeMailSend -Quiet ) {
          Write-Host "$($resource.DisplayName) $scopeMailSend has already been granted for SharePoint Online Web Client Extensibility" -ForegroundColor Green
          continue
        }
        # The scope does not yet exist; add it to the property and update it
        $resourceGrant.Scope += " $scopeMailSend"
        Update-MgOauth2PermissionGrant -OAuth2PermissionGrantId $resourceGrant.Id -Scope $resourceGrant.Scope -ErrorAction Stop | Out-Null
      }      
      # Otherwise, create a new object with the scope 
      else {
        $params = @{
          "clientId"    = $spfx.id
          "consentType" = "AllPrincipals"
          "resourceId"  = $resource.id
          "scope"       = $scopeMailSend
        }
        New-MgOauth2PermissionGrant -BodyParameter $params -ErrorAction Stop | Out-Null
      }
      Write-Host "$($resource.DisplayName) $scopeMailSend granted for SharePoint Online Web Client Extensibility." -ForegroundColor Green
}
catch {
  Write-Host "The following error occurred: $_.Exception" -ForegroundColor Red
}
finally{
    $_ = Disconnect-MgGraph # Assigning the output to a variable hides it from the terminal
}

To configure the action Send Mail additional properties should be specified:

Configuration