Using google easymail and outgoing messages blocked
How do you prevent emails being blocked (in a simple way - I'm not a techie!)
iMac 21.5″ 4K, macOS 13.2
Most emails are blocked either because the recipient is on a blacklist, or the file size of your message attachments is too large for where you are sending messages. Be certain that you didn't make a typographical error in the email addresses. There are no commas or parenthesis in emails, and some even hate apostrophes. http://www.gmail.com/ offers you access to Gmail via a web browser. Please note, since your question is in the Mountain Lion forum, the old version of the App made for Mountain Lion may no longer work. See this this page for the most compatible web browser:
Browsers by Mac operating system - Apple Community
Go to Apple menu -> About This Mac to verify your operating system. if you aren't running Mountain Lion, please repost the question in the correct forum.