SMTP Error: data not accepted

I’m using Easy WP SMTP and test emails are working fine.
Nonetheless iThemes won’t send email with database backup.

id               => 74894
module           => notification_center
type             => error
code             => send_failed::backup
timestamp        => 2019-05-18 21:45:11
init_timestamp   => 2019-05-18 21:43:13
remote_ip        => xxx.xxx.xxx.xxx
user_id          => 1
url              => https://www.website.com/wp-admin/admin-ajax.php
memory_current   => 41281184
memory_peak      => 95976464
data             => Array
    error   => Object WP_Error
        errors       => Array
            wp_mail_failed   => Array
                0   => SMTP Error: data not accepted.
        error_data   => Array
            wp_mail_failed   => Array
                to                         => Array
                    0   => [myemail]
                subject                    => Database Backup