If you have a way to contact the recipient outside of email (e.g., phone, messaging app), you can ask them to check their spam folder for your email. Ensure that your email address is recognizable to the recipient. Using a professional email address associated with your domain name can help. There are several online tools available that can check if your email address is on a blacklist. Examples include MXToolbox (https://mxtoolbox.com/blacklists.aspx) and Blacklistalert.org (http://www.blacklistalert.org/).
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