Are you running a mail server on your localhost? (Typically you should see the sendmail service running if on a Linux host.)
Look in the /opt/couchbase/var/lib/couchbase/logs/debug.log for “ns_mail” messages which should give you an indication of the problem. For example, here is the message logged when I stop my sendmail service and try to send an alert e-mail:
[user:warn,2022-07-04T07:26:35.446+01:00,ns_1@192.168.2.22:<0.24578.333>:ns_mail:do_send_async:77]Could not send email: {error,retries_exceeded,
{network_failure,"::1",{error,econnrefused}}}. Make sure that your email settings are correct.
(user1@couchbase.com will result in a delivery failure, though it should still send. Typically test with a local user to begin with so you can confirm receipt.)
HTH.