There is a way you can do this with a script, this isn’t bulletproof way, but it would save you a ton of buck.
Create a script that checks for incorrect email addresses, e.g anything written as @gimail or @ggmail would be automatically deleted, if you don’t want to delete it, you can loop your way through the list and redirect it to a file or CSV file with a name incorrect_email.csv, you get the idea or just check the mx records if it exists instead, see below.
Another super useful thing to do is to cross-check your mail list with a list of disposable email providers, if anything matches your list, delete them right away, it is disposable, and you don’t want to deal with that. Here is a list that is always updated: https://gist.github.com/michenriksen/8710649
Before checking if the actual email address exists at all, you can also create a script that pings the mail server if it’s still online, you know when you send a mail to a user, the message would first need to go to the server which then looks if MX records exist for the mail server, for example, using
dig +short gmail.com mx returns:
If it doesn’t return anything, well, you can safely assume the server is no longer in use, so, in that case, you can redirect the bad one to a file or just delete them right away.
Even if a mail server exists, it doesn’t mean that an email address exists on that mail server, it is even worst if the email address is a catch-all email alias, in which case, any email address would also be valid, but it is still better than nothing.
The last step would be to telnet the SMTP server that was returned when you used the dig command, just pick one or use one with a lower priority, and use telnet to simulate sending, then monitor the response:
250 2.1.0 OK
550-5.1.1 The email account that you tried to reach does not exist. - Gmail
452-4.2.2 The email account that you tried to reach is over quota. - Gmail
554 delivery error: dd Not a valid recipient - Yahoo
You can create a script that does these 4 things simultaneously, but don’t bombard the servers as your list is huge, so, you would want to use it with delay (maybe cron) or rotating of IPs. Like I said above, this isn’t a 100% bulletproof way, but would save you a couple of bucks
Edit: If you want to embark on this journey, you can rent a disposable VPS (5 bucks or so) for this, you don’t want to do this on your actual mail server, just a heads up.