Lots of different tactics can be employed; you can use javascript validation for emails on the front end. You can use a smart list to process all of the invalid entries on the back end (via smart campaign). For phone numbers there's tools like informatica for addressing verification on the front end. You can also use progressive profiling to manage revealing that field to only engaged prospects to improve your accuracy rate.
Be sure to check out Form Validation Rules which is a more recent Marketo Engage feature. You can use them to block certain domains from submitting your forms. There's an out-of-the-box junk domain list, or you can make your own.
Also, be sure to use "select" field types on forms whenever possible, such as for a Country field, to avoid value variations or typos that may occur when people need to type in the value.
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