How can I send German umlauts correctly with PowerShell Send-MailMessage?

The main issue in the posted example is that the splat hashtable is created but never passed to Send-MailMessage. Call it with the splatting operator, and set Encoding there:

$sendMailMessageSplat = @{
From = '[email protected]'
To = $line.Email
Subject = "$($line.DBv) $($line.Server)"
Body = 'ÄäÖöÜü'
SmtpServer = 'smtp.mail.com'
Encoding = 'UTF8'
}

Send-MailMessage @sendMailMessageSplat

Variables work in a normal double-quoted body string, and they work in an HTML body as well. Just use BodyAsHtml = $true and construct the HTML as a string, for example: Body = "

Hello $($line.Name)

".

A minimal test looks like this:

$params = @{
From = '[email protected]'
To = '[email protected]'
SmtpServer = 'smtp.example.com'
Subject = 'Test with ä'
Body = 'This is a test: ÄäÖöÜü'
Encoding = 'UTF8'
}
Send-MailMessage @params

Also, avoid rewriting the CSV just to add headers if possible. Import-Csv supports a Header parameter, which reduces the chance of changing the file’s encoding accidentally.

If the CSV is involved, use the same known encoding when reading and writing it. In Windows PowerShell 5.1, saving the script as UTF-8 with a BOM can also prevent hardcoded non-ASCII characters from being misread. For example:

$encoding = 'UTF8'
$P = Import-Csv -Path $CSVLocation -Delimiter ',' -Encoding $encoding

Then use Encoding = $encoding in the Send-MailMessage parameters. If the umlauts are hardcoded and the CSV does not contain them, Send-MailMessage’s encoding and the script file’s encoding are the important parts.

Replacing ü with ue is a possible German-language fallback, but it isn’t necessary if the message is encoded correctly. UTF-8 should preserve the original characters. One longer-term consideration is that Send-MailMessage is deprecated, so a mail service API or another supported SMTP solution would be safer when you have the option to replace it.