SMTP error "StartTLS not supported by server or could not initiate session"

Issue

Email services routed through the SMTP module can encounter a timeout error if the SMTP service is unavailable or using TLS version 1.0.

Example error:

phpmailerException: StartTLS not supported by server or could not initiate session. in PHPMailer->SmtpConnect()

You can test the timeout behavior by SSH'ing into your server to run the following command, replacing the SITENAME with your application's site name and ENV with the desired environment:

SITENAME@srv-1234:/var/www/html/SITENAME.ENV/docroot$ openssl s_client -connect www.example.com:25 -starttls smtp
connect: Connection timed out
connect:errno=110

It's also a good idea to double check the module variables:

SITENAME@srv-1234:/var/www/html/SITENAME.ENV/docroot/sites/default$ drush vget smtp --uri=https://example.com | egrep "port|protocol|from|host"
smtp_client_hostname: ''
smtp_from: [email protected]
smtp_fromname: 'NAME OF SITE'
smtp_host: smtp-relay.example.com
smtp_hostbackup: ''
smtp_port: '25'
smtp_protocol: tls

Resolution

Contact your SMTP service to check for any service-related issues, as well as confirm the SMTP server being used has TLS version 1.2 and above. Please note support for TLS version 1.0 is deprecated on the Acquia platform.