Notification Email could not be sent out

Hi,
I’ve finished the configuration of SMTP, then do the test_email as the gitlab docs guiding.

I can receive the test mail sent at the test_email step, but I couldn’t get any notification email when create account for new user, or create an issue.

Then I check the Background Jobs information in the Admin Area. It shows the ActionMailer::MailDeliveryJob in the mailers failed.

The job parameter is:

"DeviseMailer", "confirmation_instructions", "deliver_now", {"args"=>[{"_aj_globalid"=>"gid://gitlab/Email/4"}, "v8jYTRgVorJNcQk3KMEe", {"_aj_symbol_keys"=>[]}], "_aj_ruby2_keywords"=>["args"]}

The error log is:

Net::ReadTimeout: Net::ReadTimeout with #<TCPSocket:(closed)>

How should I debug and correct my setting next?
Hoping for your help! Thanks very much!