You can't know if your IP is already tainted until you realize your mails are landing in people's spam box. I use one of the service even for my private email

You can't know if your IP is already tainted until you realize your mails are landing in people's spam box. I use one of the service even for my private email server (which is essentially free for the volume being sent), so I don't have to have stupid conversation like check for spam box whenever I send emails. Having emails land in spam box pretty much makes your email non existent with the volume of spams being sent today. What is the reason not to use one of the services either privately or professionally?