1st: Ideal Scenario: Generally all Emails and Mobile Numbers getting fed into Organisation DB through a form it can be lead form, Application form on the website. There should be a mechanism that customer should verify the email and mobile before submitting the form.
2nd Many times companies are reluctant to implement double opt-in (eg, OTP Verification) as there can be a major drop off chances. In this case, Adobe will receive junk email IDs and that impacts the overall deliverability of subdomain and IPs. In this case, there multiple vendors who provide service to identify the invalid email id but that is paid services.
The only problem for Adobe Campaign is we cannot decide the email ID as invalid unless we send some emails and we get hard bounce data. It is better to identify the invalid email IDs before we load the data in any campaign management tool else implement the strong double opt-in strategy on the websites.