Allow for unauthenticated SMTP. (#81)

* Allow for unauthenticated SMTP, fixes #66

backup.ps1

@@ -302,8 +302,12 @@ function Send-Email {
         $Action = "Backup"
     }
 
-    $password = ConvertTo-SecureString $ResticEmailPassword -AsPlainText -Force
-    $credentials = New-Object System.Management.Automation.PSCredential ($ResticEmailUsername, $password)
+    if ([String]::IsNullOrEmpty($ResticEmailPassword)) {
+        $ResticEmailCredentials = @{}
+    } else {
+        $password = ConvertTo-SecureString $ResticEmailPassword -AsPlainText -Force
+        $ResticEmailCredentials = @{Credential = New-Object System.Management.Automation.PSCredential ($ResticEmailUsername, $password)}
+    }
 
     $status = "SUCCESS"
     $past_failure = $false
@@ -334,7 +338,7 @@ function Send-Email {
         # create a temporary error log to log errors; can't write to the same file that Send-MailMessage is reading
         $temp_error_log = $ErrorLog + "_temp"
 
-        Send-MailMessage @ResticEmailConfig -From $ResticEmailFrom -To $ResticEmailTo -Credential $credentials -Subject $subject -Body $body @attachments 3>&1 2>> $temp_error_log
+        Send-MailMessage @ResticEmailConfig -From $ResticEmailFrom -To $ResticEmailTo @ResticEmailCredentials -Subject $subject -Body $body @attachments 3>&1 2>> $temp_error_log
 
         if(-not $?) {
             "[[Email]] Sending email completed with errors" | Tee-Object -Append $temp_error_log | Out-File -Append $SuccessLog