It is always recommend to clean the list every 3 months once before sending an email campaign.
In BounceKick email verification software by default the emails are automatically get verified for every 90 days once and updated the existing status. When ever you would like to send an email campaign you can just download the list and trigger the campaign.
I would advise cleaning you lists every 3 months to maintain and good clean database that will perform well when carrying out email marketing and help maintain and good sending reputation.
You can clean your lists using an industry solution like Email Checker (www.emailchecker.com)
You can create a free account here: login.emailchecker.com and you will also get 100 free credits to test with.
I would highly recommend you to scan the list and try sending the emails only to the valid addresses. As a general recommendation list need to be cleaned once in three months of time.