PowerShell Email Works Manually but Fails in Task Scheduler

The Problem

I was working with a PowerShell automation that generated a SharePoint Online site consumption report and emailed the resulting CSV file.

When I ran the script manually from PowerShell 7, everything worked. The SharePoint inventory was collected, the CSV was generated, and the email arrived successfully.

However, when the same script was executed through Windows Task Scheduler, something strange happened.

• The scheduled task completed.
• SharePoint Online authentication succeeded.
• The site inventory was generated.
• The CSV file was created.
• The script reached the email section.
• Send-MailMessage reported success.
• Task Scheduler reported 0x0.
• But the email did not arrive.

First: Was the SharePoint Report Actually Completing?

Before troubleshooting SMTP, I needed to determine whether the SharePoint portion of the script was actually completing.

The scheduled execution successfully retrieved more than 32,000 SharePoint sites and generated the final CSV report.

Connecting to SharePoint Admin Center...
Connected successfully.

Retrieving SharePoint site inventory...

Total Sites Found: 32611

Processing SharePoint sites...
Throttle Limit    : 3

Results Count = 32611

Exporting final CSV...

CSV created:
Mattel-SPOSiteConsumption_20260925_070755.csv

The report summary was also generated successfully.

Total Sites          : 32611
Storage Used         : 526.07 TB
Allocated Storage    : 159.51 TB
Over Utilized        : 366.56 TB
Over Utilized %      : 229.80%

This was important because it eliminated SharePoint connectivity and report generation as the primary cause.

Testing the Email Function Independently

The next step was to remove SharePoint completely from the equation.

I used the already-generated CSV and manually tested the same sender, SMTP server, recipient, and attachment.

The email arrived successfully.

A Recipient Formatting Mistake

During troubleshooting, I temporarily added an individual email address alongside the distribution list.

Initially, I used this format:

$To = @(
    "messaging@example.com,user@example.com"
)

This looks reasonable at first glance, but it actually creates one array element containing a comma-separated string.

PowerShell then produced:

An invalid character was found in the mail header: ','.

The correct format is to provide each recipient as a separate array element:

$To = @(
    "messaging@example.com"
    "user@example.com"
)

This was a useful finding, but it still wasn't the underlying Task Scheduler delivery problem.

The Important Clue: Execution Context

At this point, I compared how the script was being executed manually versus how Windows Task Scheduler was executing it.

This revealed a significant difference.

The working automation on the same server was configured to use a dedicated service account rather than SYSTEM.

Why does the execution account matter?

A PowerShell process launched interactively by an administrator does not necessarily have the same execution context as a process launched by Task Scheduler under SYSTEM.

The execution identity can affect:

• Network access
• Authentication context
• SMTP relay permissions
• Security policies
• Certificate and private-key access
• File-system permissions
• Proxy configuration
• User-specific environment settings

The Controlled Task Scheduler Test

Rather than repeatedly running a SharePoint inventory containing tens of thousands of sites, I created a very small PowerShell script that only sent an email.

$From = "report@example.com"
$To = "messaging@example.com"
$SmtpServer = "smtp.example.com"

$Body = @"
Task Scheduler SMTP Test

Computer : $env:COMPUTERNAME
User     : $([System.Security.Principal.WindowsIdentity]::GetCurrent().Name)
Time     : $(Get-Date)

This is a controlled SMTP test.
"@

Send-MailMessage `
    -From $From `
    -To $To `
    -Subject "Task Scheduler SMTP Test" `
    -Body $Body `
    -SmtpServer $SmtpServer `
    -ErrorAction Stop

I then configured a temporary Task Scheduler task to run this small test using the same PowerShell version as the production automation, but under the dedicated service account.

The test email arrived successfully.

Resolution

The production SharePoint reporting task was changed from the SYSTEM account to the appropriate dedicated service account.

The PowerShell 7 action and the SharePoint reporting script itself were left unchanged.

The recipient configuration remained simple:

$To = @(
    "messaging@example.com"
)

After the execution account was corrected, the scheduled automation was able to submit the email successfully and the distribution list received the report.

Lessons Learned

1. Task Scheduler success does not mean email delivery success

A Task Scheduler result of 0x0 indicates that the scheduled task completed successfully. It should not be treated as proof that an email reached the recipient's mailbox.

2. "Email sent successfully" is not end-to-end delivery confirmation

A successful Send-MailMessage call indicates that the SMTP operation completed from the PowerShell process's perspective. Downstream mail-flow processing can still affect final delivery.

3. Test SMTP separately from SharePoint

If a SharePoint report takes hours to generate, don't repeatedly execute the entire report just to test email delivery.

A small SMTP test script can isolate the email path in seconds.

4. Always compare execution identities

When a script behaves differently between manual execution and Task Scheduler, check the identity actually running the process.

[System.Security.Principal.WindowsIdentity]::GetCurrent().Name

5. Don't overlook the Task Scheduler Action

The account isn't the only thing that can differ. Also compare:

• PowerShell version
• Program/script path
• Arguments
• Start in directory
• Environment variables
• Run whether user is logged on or not
• Run with highest privileges

Troubleshooting Checklist

1. Run the PowerShell script manually.
2. Confirm that the SharePoint operation completes.
3. Confirm that the expected CSV/report is generated.
4. Send the generated attachment manually.
5. Test SMTP without involving SharePoint.
6. Check the Task Scheduler execution account.
7. Compare the PowerShell version used by manual and scheduled execution.
8. Create a minimal SMTP test under the exact Task Scheduler account.
9. Test the distribution list separately from the full automation.
10. Only after identifying the difference should you modify the production task.