Step By Step Guide:
1. Go SmarterMail mailbox, cancel mail forwarding and cancel mail deletion.
2. Go Gmail Setting at "Account and Import"
3. Go "Check mail from other accounts (using pop3)" to "Add a POP3 mail account you own"
4. Add in
5. If denied, try again, if gmail found same domain may need to try again....
6. Tick "Yes, I want to be able to send mail as..."
7. Name: advisable to enter full email address
8. Advised "NOT" to tick "Treat as an alias", go click "Specify a different "reply-to" address, and enter your full email address, click "Next Step"
9. SMTP server enters "mail
10. Username enter full email address and password
11. Tick "Secured connection using TLS", click "Add“
12. Verify mail from SmarterMail (or your email service provider) mailbox
13. Enter "verify confirmation code" to verify
14. Go to Gmail/Setting/Label, at "label" tab, if the label exists, select edit whether to nest the label to "Parent"? Or "create new label", advisable as full email address (if multiple email under a domain, advisable create a parent label)
15. Go Gmail/Setting, "Filters" tab
16. At bottom "Create a new filter"
17. At "To" add in the particular filter mail address, click button "create filter with this search"
18. Tick "apply the label": the label that you created just now.
11. Tick "also apply
12. Click button
Relevant links:
https://support.google.com/mail/answer/21289?hl=en