Alternatively, pick up the phone, call the company they work for, and ask for their email. "Hi, I'm <x>, I'm trying to send an important email to <y> but it ke

Alternatively, pick up the phone, call the company they work for, and ask for their email. "Hi, I'm <x>, I'm trying to send an important email to <y> but it keeps bouncing. Can you confirm I've got their email address correct?"