DO NOT send 20k or even 10k emails through your business email account.
Someone will very likely get you flagged as a spammer and you may end up on blacklists.
If your ISP catches it they may shut you down.
Just not a good idea in general and bad for business.
You need a legit mass email service.
Mailchimp is free for 2,000 contacts and 12k emails sent per month.
Send Better Email | MailChimp
So what you can do is load up 2k names, send to that list, delete the list, load another 2k names, send, rinse and repeat until you hit the 12k limit.
You could sign up under two separate email addresses and get 24k in a month that way.
Or you could pay $25 to get unlimited emails for the month but you'd still need to do the list rotation.
20k names in the list will run you $155 for the month so you could look at it as a one time fee.
Now the thing with email addresses is they tend to expire, don't be surprised if that 20k is more like 15k or less.
Who is your ISP?
Often as part of your hosting package they will provide a way for you to send mass email campaigns.
Check that first.