Deprecated notification

Hello,
I’m using the latest WordPress and your plugin versions.
I’m also using latest Gravity Forms and a couple of extensions.
When submitting a form, the user is given a confirmation on the front end and behind the scenes GF sends a notification email.
On the confirmation page this notification shows up. Mind you, everything seems to be working. Just this notice: (“mywebsite” added for obvious reasons)

Deprecated: implode(): Passing glue string after array is deprecated. Swap the parameters in /home/mywebsite/public_html/wp-content/plugins/post-smtp/Postman/Postman-Mail/google-api-client/vendor/google/apiclient/src/Google/Service/Resource.php on line 291

Deprecated: implode(): Passing glue string after array is deprecated. Swap the parameters in /home/mywebsite/public_html/wp-content/plugins/post-smtp/Postman/Postman-Mail/google-api-client/vendor/google/apiclient/src/Google/Service/Resource.php on line 291

I’ve not yet seen it any other place.