Could not open socket

HostName: cantinafratello.com.br
OS: Linux us-imm-web106.main-hosting.eu 2.6.32-896.16.1.lve1.4.53.el6.x86_64 #1 SMP Sun Feb 18 08:20:42 EST 2018 x86_64
PHP: Linux 7.0.31 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.9.8 pt_BR UTF-8
WordPress Theme: Restaurante Catina Fratello
WordPress Plugins: AddToAny Share Buttons, Coming Soon Page & Maintenance Mode by SeedProd, Contact Form 7, Food and Drink Menu, Post SMTP, Restaurant Reservations, Sidebar Login, Simple Image Sizes, WP User Avatar, WP-UserOnline, WPFront Scroll Top, NextGEN Gallery
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init
Postman: 1.9.4
Postman Sender Domain (Envelope|Message): gmail.com | gmail.com
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SMTP (smtps:oauth2://smtp.gmail.com:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 56 | 5

I received this error:

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

How can I solve this?

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