ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.

Right Inbox - Schedule Emails in Gmail To Be Sent Later

clearout.io - Clearout Email Finder & Email Verification Service - Drive sustainable business growth by expanding your reach, lowering hard bounces, and improving deliverability rates.

Recurrr - Send recurring emails

DeBounce - Email Validation, Email Checker, Data Enrichment and Appending Tool. Using DeBounce remove invalid, disposable, spam-trap, syntax and deactivated emails.

FollowUpThen - The simplest way to schedule an email reminder