This  issue is caused by a change in Gmail security policy. Please use the following steps to workaround this problem:

- Sign into Gmail using your web browser
- In the top right corner select the google apps button (the button that is made up of 9 squares)
- Select “My Account”
- Select “Sign-in & Security”
- Scroll down until you come to the “Connected apps & sites”
- Look for the “Allow less secure apps” option and turn that to ON
- Go back to ACDSee and send your e-mail

Please also use the following to Gmail Server settings in ACDSee

Server: smtp.gmail.com
Port : 587
Turn ON the option "Server Require authentication"
Turn ON the option "This Server requires an encrypted connection (SSL)