Outgoing sales invoice emails very slow or not working

We are also having this issue...... We email from MYOB through outlook and it can take sometimes 10min for the email to go and sometimes it doesn't. Emailing payslips & Statements can sometimes be a nightmare, but the individual Invoice or Quotes the wait between email and being able to do the next one is ridiculous. My IT guys have contacted MYOB who say it's not their issue and Microsoft say the same. My IT guys have exhausted all obvious options....