SMTP Error: 454 4.7.0 Too many login attempts, please try again later

I have set up SMTP server with gmail account. It was working fine till few days back. When I checked the logs I found below entry in it:

SMTP Error: 454 4.7.0 Too many login attempts, please try again later.

I have restarted SMTP service twice. I have checked the configuration that was set up using this link. Everything is same as we have set up. I have restarted the SMTP server & the machine too.

I have checked for 2 step verification settings. It is not enabled. I have checked for "less secure" apps settings and it is set to Enabled as suggested here.

I have checked apps enabled as suggested here using below link.

https://security.google.com/settings/security/permissions?pli=1

But no apps are added. Can anyone suggest anything that I need to look for? Thanks in advance.

The Issue resolved with the TCP port changing to 587 from 25 in Outbound Connections settings in SMTP Server.