Handling Neverbounce result codes

Question

Hi,I had a question regarding Neverbounce results. I'm running a list of some leads through Neverbounce to see how many are not valid and take appropriate action. I see Neverbounce returns a lot of emails that fall in the Unverifiable and Unknown categories. I wanted to know as a best practice, how do you handle these result codes? Do you only keep the emails marked 'Valid' in the database? What's an appropriate course of action for the Unverifiable and Unknown categories of emails?  Thanks,Shweta.

SanfordWhiteman · Answer

Any email verification service — I say this from direct experience building one — will have some unclassifiable results. Those should not be preemptively considered to be invalid addresses for Marketo sends.*

There are a few reasons for this:

the email verification service may be rate-limited by IP while your actual Marketo instance is not
the remote mailserver is known to say any address is valid during a RCPT TO check (this is the only check a verification service can safely perform) but rejects after DATA or later; for such mailservers, everything is unclassifiable but that doesn’t mean it’s bad
the remote mailserver employs a form of anti-spam greylisting that for technical reasons means the email verifier never gets a firm answer

* Of course the converse is also true: just because an email address is valid in general doesn’t mean Marketo in particular can successfully send to it.

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