Right now mail responses goes to SPAM

Content

Consider i have two types of user accounts.

one user account with email address(User account 1 ) - [email protected]

Another user account with email address(user account 2 ) - [email protected]

The responses sent from User account 1 goes to inbox folder where as the responses sent from User account 2 goes to SPAM folder.

i need suggestions on sorting out this issue.

Version

Service cloud