Providers

Any of the following services should work well for you:

http://www.accuwebhosting.com/blog/2014/09/29/top-10-email-list-validation-verification-services-compared/
https://www.marketingtechblog.com/verify-email-lists/
Usually 1 penny a contact.
We suggest:

Neverbounce
BriteVerify (easiest to use if you're in a pinch)
TowerData (strong quality)
Experian (formerly LeadSpend)
Kickbox.io

Typical Response/Result Codes

Depending on the prov

All email validation providers will provide anywhere from 3-5 response codes. For example, let's look at NeverBounce. They have 5 response codes, as shown below. All providers will have very similar, if not identical, response codes. As NeverBounce discusses on this page, and in the table below, only the "valid" response codes are good to send to.

Only Send to Contacts with valid as the Response Code.

Text Code	Numeric Code	Description	Safe to Send?
valid	0	Verified as real address	Yes
invalid	1	Verified as not valid	No
disposable	2	A temporary, disposable address	No
catchall	3	A domain-wide setting (learn more)	Maybe Not recommended unless on private server
unknown	4	The server cannot be reached	No

Validating New Prospects

Validating Existing Prospects

Follow these instructions to validate email addresses currently inLead Liaison. As a best practice, if your business is B2B we suggest email validation every 6 months. If your business is B2C, we suggest email validation every year.

Do a simple export of only the email, name, and company. Email is the minimum requirement. Exporting more than this will just increase time to export/process in Lead Liaison as all other fields are superfluous for the purpose of validating email addresses. Save the file as a CSV.
Use your original CSV from Step #1 and validate the emails using your 3rd party of choice.
Generate a new CSV file (must import CSV into Lead Liaison) with two columns, Email and "Is Valid" (or something similar).
Reduce the CSV to only invalid emails, removing the good emails.
Import the CSV, map the Email field to the Lead Liaison Email field, and the "Is Invalid" field to the "Do Not Email" field in Lead Liaison. Note, when you import, you have to import into a List. If you have a general List with all contacts you can simply re-import into the same List without creating a new List; otherwise, you'll need to create a new List and import into it.
If you want to delete bad records after your import is done then setup a process to delete all prospects with the Do Not Email field equal to true. Deleting a Prospect also removes them from any Lists they are part of. This step is optional though. You can keep the contacts with bad emails in the system if you like. Keeping them in the system will prevent emailing the same bad emails in the future.