Could not open socket

Hi, we are on Digital Ocean using your plugin version: 1.9.8, domain mail smtp is smtp.gmail.com:587, delivered almost 20k mail without any error… your plugin is working great congrats!!!

A few days ago we had a mail reporting:

You getting this message because an error detected while delivered your email.
For the domain: https://www.cuchillalia.com
The log to paste when you open a support issue:
Could not open socket

We checked the connectivity test and all is ok, can be an error like you wrote here? https://postmansmtp.com/post-smtp-1-9-4-im-only-human/
do we have a bad configuration or simply was gmail that closed a port for a while?

thank you very much
Angelo

The page I need help with: [log in to see the link]