You are telling that you have thousands of email lists. If you have this big data you have to be more careful about email deliverability. If you don't verify before sending, it is very sure that there are some invalid and unuseful email accounts that increase your bounce rate. Continuously bounce rate affects your sender score and hurts your domain reputation.
So it is recommended to verify your list before sending an email. Email verifiers help you catch undelivered emails also spam traps, role-based emails, disposable emails, syntax checks, etc.
There are many email verifiers in the market which help you verify your email list. You can choose the best email verifyers by its accuracy, turnaround time, cost, data protection, and customer support.
If you have further query, visit check Top 10 Email Verifiers list.
First of all, you need to find the best email service provider who can carry your bulk emails for you. Some leading ESPs are MailChimp, Constant contact, Drip, Aweber etc. Currently, I am using MailChimp. Quite happy with that. I found the best comparison of ESPs on accuwebhosting blog. You can easily compare all of these providers feature wise.
Apart from that, you must clean your mailing list before sending mass emails. Email Verification is the process of using a specialized software that checks if the email address on your mailing list is safe to send, i.e. Valid or not. Some of the leading email verification service providers that validate email addresses are real or not. They use different techniques to validate catch-all accounts, role, and disposable email addresses etc. Some of the leading email verification service providers are zerobounce, Quickemailverification, Xverify etc.