If you use some servers that have SolusVM that support IPv6 and get the problem could not send mail to Gmail when using Exim or other scripts to install a complete VPS.
The simple solution is to disable IPv6.
Let open file /etc/sysctl.conf
nano /etc/sysctl.conf
Adding to end of the file with lines as follows:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
Save and reload settings.
sysctl -f /etc/sysctl.conf
Now it is done, the system disabled the IPv6. Good luck!