Comprehensive Guide to PowerShell Send-MailMessage with Multiple Recipients

Problem Background and Error Analysis

In PowerShell script development, using the Send-MailMessage command for email automation is a common requirement. However, when sending emails to multiple recipients, many developers encounter the issue where only the first recipient receives the message. This typically stems from misunderstandings about parameter types.

Consider the following typical erroneous code example:

$recipients = "Marcel <marcel@turie.eu>, Marcelt <marcel@nbs.sk>"

Get-ChildItem "C:\Decrypted\" | Where {-NOT $_.PSIsContainer} | foreach {$_.fullname} |
send-mailmessage -from "primasfrb@nbs.sk" `
            -to "$recipients" `
            -subject "New files" `
            -body "$teloadmin" `
            -BodyAsHtml `
            -priority  High `
            -dno onSuccess, onFailure `
            -smtpServer  192.168.170.61

In this code, the $recipients variable is defined as a single string containing multiple email addresses separated by commas. While this may appear reasonable superficially, the Send-MailMessage command's -To parameter actually expects a string array (string[]), not a single string with delimiters.

Core Solution Principles

PowerShell's type system and parameter binding mechanism play crucial roles in this issue. When the -To parameter receives a string array, PowerShell properly processes each array element as an independent recipient address. However, when receiving a single string, PowerShell attempts to treat it as a complete recipient identifier, causing subsequent addresses to be ignored.

The correct solution involves ensuring the $recipients variable contains a genuine array. Here are several effective implementation approaches:

Method 1: Explicit Type Declaration

Using explicit type declaration ensures the variable always contains the correct data type:

[string[]]$recipients = "Marcel <marcel@turie.eu>", "Marcelt <marcel@nbs.sk>"

This approach provides compile-time type checking, avoiding runtime type conversion errors.

Method 2: Implicit Array Creation

PowerShell's flexible type system allows array creation through simple assignment:

$recipients = "Marcel <marcel@turie.eu>", "Marcelt <marcel@nbs.sk>"

While this creates an object[] type array, PowerShell's type coercion rules properly handle the conversion to string[].

Method 3: Array Operator Usage

Explicitly create arrays using the @() operator:

$recipients = @("Marcel <marcel@turie.eu>", "Marcelt <marcel@nbs.sk>")

This method offers optimal code readability, clearly expressing developer intent.

In-Depth Technical Analysis

Understanding the root cause of this issue requires delving into PowerShell's parameter binding mechanism. The Send-MailMessage command's -To parameter is defined to accept string[] type, meaning it expects a string array.

When a single string is passed, PowerShell attempts type conversion. Since the string itself is not an array, the conversion process fails or produces unexpected results. At the SMTP protocol level, this causes only the first valid email address to be recognized and processed.

The error message mentioned in the reference article, "Cannot process argument transformation on parameter 'msgrecipient'. Cannot convert value to type System.String," further confirms the type conversion issue. This indicates PowerShell encountered difficulties converting complex object structures to simple strings.

Complete Solution Implementation

Based on the above analysis, here is the complete correct implementation:

# Define recipient array
[string[]]$recipients = "Marcel <marcel@turie.eu>", "Marcelt <marcel@nbs.sk>"

# Get file list and send emails
Get-ChildItem "C:\Decrypted\" | 
Where-Object {-NOT $_.PSIsContainer} | 
ForEach-Object {$_.FullName} |
Send-MailMessage -From "primasfrb@nbs.sk" `
                 -To $recipients `
                 -Subject "New files" `
                 -Body $teloadmin `
                 -BodyAsHtml `
                 -Priority High `
                 -DeliveryNotificationOption onSuccess, onFailure `
                 -SmtpServer "192.168.170.61"

Best Practices and Extended Applications

Beyond basic array definition, the following best practices can further enhance code robustness and maintainability:

1. Dynamic Recipient Management
Dynamically load recipient lists from external files or databases:

$recipientList = Get-Content "recipients.txt"
[string[]]$recipients = $recipientList -split ","

2. Attachment Array Handling
The same array principle applies to attachment parameters:

$attachments = @("$PSScriptRoot\image003.png", "$PSScriptRoot\image004.jpg")
Send-MailMessage -To $recipients -Attachments $attachments # Other parameters omitted

3. Error Handling
Add appropriate error handling mechanisms to ensure email sending reliability:

try {
    Send-MailMessage @mailParams -ErrorAction Stop
    Write-Host "Email sent successfully" -ForegroundColor Green
} catch {
    Write-Error "Email sending failed: $($_.Exception.Message)"
}

Conclusion

The multiple recipient configuration issue with PowerShell's Send-MailMessage command is fundamentally a type system problem. By correctly using string arrays instead of single strings with delimiters, developers can ensure all recipients properly receive emails. The three main array creation methods each have their advantages, allowing developers to choose the most suitable approach based on specific requirements. Understanding PowerShell's type coercion rules and parameter binding mechanism is essential for avoiding such issues.