SMTP error ” does not appear to be a valid URI hostname

hi,

I configured POSTSMTP with a external mail server. I set smpt host and when I send test email I get this error :

'' does not match the expected structure for a DNS hostname, '' does not appear to be a valid URI hostname, '' does not appear to be a valid local network name

it’s strange, like it doesnot take my smtp host configuration.

WP : v4.9.5
POST SMTP : v1.8.9

here is the configuration :
SMTP-STARTTLS à ssl0.ovh.net:587 en utilisant l’authentification Mot de passe (Login).

Please help !