$File = "C:\ps\mail_pwd.txt"
$FileBody = "C:\ps\mail_body.txt"

$UserName = "login@yandex.ru"
$PlainPassword = "password"

$PlainPassword | ConvertTo-SecureString -AsPlainText -Force | ConvertFrom-SecureString | Out-File $File
$SecurePassword = Get-Content $File | ConvertTo-SecureString

$Creds = New-Object –TypeName System.Management.Automation.PSCredential -ArgumentList $UserName, $SecurePassword

$EmailFrom = "login@yandex.ru"
$EmailTo = "login2@yandex.ru"
$Subject = "тест"
$Body = Get-Content $FileBody

$SMTPServer = "smtp.yandex.ru"
$SMTPClient = New-Object Net.Mail.SmtpClient($SmtpServer, 587)
$SMTPClient.EnableSsl = $true

### Так работает
### $SMTPClient.Credentials = New-Object System.Net.NetworkCredential($Creds.UserName, $PlainPassword)

### Так не работает
$SMTPClient.Credentials = New-Object System.Net.NetworkCredential($Creds.UserName, $Creds.Password)

$SMTPClient.Send($EmailFrom, $EmailTo, $Subject, $Body)

$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($SecurePassword)
$UnsecurePassword = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)