Here I am with some quick solution, if you are sending email connecting to
external smtp server like smtp.gmail.com you may have to consider to check 2
1. Check smtp ports are open or make sure its not block:
Open your WHM and go to "ConfigServer Security & Firewall" and Edit the configuration file for the csf firewall and lfd or click on "Firewall Configuration" locate SMTP_PORTS and make sure porst are open (Ex: 25,465,587)
2. Check smtp allowed users are added in the list or Always allow the following comma separated users and groups to bypass SMTP_BLOCK
Open your WHM and go to "ConfigServer Security & Firewall" and Edit the configuration file for the csf firewall and lfd or click on "Firewall Configuration" locate SMTP_ALLOWUSER and add the cpanel username (Ex: cpanel, )
Save the changes & you restart both csf and lfd.
In anyway if this one helps you, please make a comment & share in your post
This is really strange, new problem rise everyday and we have to find solution, codes are also same like life now.
“Password not accepted from server: 535 Incorrect authentication data” when sending with GMail and phpMailer
using CentOs 6 Login to CPanel > Tweak Settings > All> "Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak)" <== disable it.