How do you the emails of apartment owners?

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Hello, 

Just watched the podcast with Axel about his off-market strategies on apartment complexes. One strategy I couldn’t quite understand how he did efficiently is his email campaign. How did he find and validate the emails for these apartment complex owners? I assume he didn’t get the email for each and every property but has to for a majority of them if this was a big marketing campaign for him. 

Thank you!