Добавьте Mail Converter Pro в ваше решение

public static class Function1
    {
        [FunctionName("Function1")]
        public static async Task Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            StringBuilder sbLogs = new StringBuilder();
            sbLogs.AppendLine("started...");
            try
            {
                ProcessStartInfo startInfo = new ProcessStartInfo();
                startInfo.CreateNoWindow = true;
                startInfo.UseShellExecute = false;
                var assemblyDirectoryPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
                assemblyDirectoryPath = assemblyDirectoryPath.Substring(0, assemblyDirectoryPath.Length - 4);

                var executablePath = $@"{assemblyDirectoryPath}\Converter\MailConverterX.exe";
                sbLogs.AppendLine(executablePath + "...");
                var srcPath = $@"{assemblyDirectoryPath}\src\sample.msg";
                var outPath = Path.GetTempFileName() + ".pdf";
                startInfo.FileName = executablePath;

                if (File.Exists(outPath))
                {
                    File.Delete(outPath);
                }

                if (File.Exists(executablePath) && File.Exists(srcPath))
                {
                    sbLogs.AppendLine("files exists...");
                }
                else
                    sbLogs.AppendLine("EXE & source files NOT exists...");
                startInfo.WindowStyle = ProcessWindowStyle.Hidden;
                startInfo.Arguments = $"{srcPath} {outPath} -cPDF -attach -docs";
                using (Process exeProcess = Process.Start(startInfo))
                {
                    sbLogs.AppendLine($"wait...{DateTime.Now.ToString()}");
                    exeProcess.WaitForExit();
                    sbLogs.AppendLine($"complete...{DateTime.Now.ToString()}");
                }

                int sleepCounter = 10;

                while(!File.Exists(outPath) && sleepCounter > 0)
                {
                    System.Threading.Thread.Sleep(1000);
                    sbLogs.AppendLine("sleep...");
                    sleepCounter--;
                }
                if (File.Exists(outPath))
                    sbLogs.AppendLine("Conversion complete successfully.");
            }
            catch (Exception ex)
            {
                sbLogs.AppendLine(ex.ToString());
            }

            return new OkObjectResult(sbLogs);
        }
    }

dim C
Set C=CreateObject("MailConverterPro.MailConverterX")
C.Convert "c:\test\source.eml", "c:\test\dest.pdf", "-cPDF -attach -docs -log c:\Mail.log"
Response.Write C.ErrorMessage
set C = nothing

dim C
Set C=CreateObject("MailConverterPro.MailConverterX")
Response.Clear
Response.AddHeader "Content-Type", "binary/octet-stream"
Response.AddHeader "Content-Disposition", "attachment; filename=test.pdf"
Response.BinaryWrite C.ConvertToStream("C:\www\ASP\Source.eml", "C:\www\ASP", "-cpdf -attach -docs -log c:\html.log")
set C = nothing

$src="C:\\test\\test.msg";
$dest="C:\\test\\test.pdf";
if (file_exists($dest)) unlink($dest);
$c= new COM("MailConverterPro.MailConverterX");
$c->convert($src,$dest, "-cPDF -attach -docs -log c:\\Mail.log");
if (file_exists($dest)) echo "OK"; else echo "fail:".$c->ErrorMessage;

require 'win32ole'
c = WIN32OLE.new('MailConverterPro.MailConverterX')

src = "C:\\test\\test.eml"
dest = "C:\\test\\test.pdf"

c.convert(src, dest, "-cPDF -attach -docs -log c:\\test\\Mail.log")

if not File.exist?(dest)
  puts c.ErrorMessage
end

import win32com.client
import os.path

c = win32com.client.Dispatch("MailConverterPro.MailConverterX")

src  = "C:\\test\\test.eml"
dest = "C:\\test\\test.pdf"

c.convert(src, dest, "-cPDF -attach -docs -log c:\\test\\Mail.log")

if not os.path.exists(dest):
    print(c.ErrorMessage)

uses Dialogs, Vcl.OleAuto;

var
  c: OleVariant;
begin
  c := CreateOleObject('MailConverterPro.MailConverterX');
  c.Convert('c:\test\source.eml', 'c:\test\dest.pdf', '-cPDF -attach -docs -log c:\test\Mail.log');
  if c.ErrorMessage <> '' then
    ShowMessage(c.ErrorMessage);
end;

var c = new ActiveXObject("MailConverterPro.MailConverterX");
c.Convert("C:\\test\\source.msg", "C:\\test\\dest.pdf", "-cPDF -attach -docs");
if (c.ErrorMessage != "")
  alert(c.ErrorMessage)

use Win32::OLE;

my $src  = "C:\\test\\test.eml";
my $dest = "C:\\test\\test.pdf";

my $c = CreateObject Win32::OLE 'MailConverterPro.MailConverterX';
$c->convert($src, $dest, "-cPDF -attach -docs -log c:\\test\\Mail.log");
print $c->ErrorMessage if -e $dest;