To connect the Gmail mail collector to mailbox do the following:
In the main Gmail window, click on the gear button and select “
Settings»:

Switch to the "
Accounts and imports"And opposite"
Receive mail from other accounts»Click«
Add mail account»:

Pleaseindicate
name mailbox and click "
Further»:

Make sure the switch is checked against "
Import emails from another account (POP3)", and press "
Further»:

Fill in the fields and click "
Add account»:

If you need to be able not only to receive letters, but also to send, make sure that the switch is checked next to “
Yes, I want to be able to send emails from admin@example.com", and press "
Further»:

Fill in the fields and click "
Further»:

Fill in the fields and click "
Add account»:

Open pluggable
mailbox in
WebMail and follow the link from the confirmation email, or copy the code from the email, paste it into the field and click "
Confirm»:

Test your mail through the Gmail interface.
If you are setting up a mail collector not in your regular Gmail mailbox, but in a domain mailbox in Google Workspace (G Suite), then it may not be allowed to use the outgoing mail gateway:
In this case, you should contact the administrator of the Google Workspace (G Suite) account to which your Google mailbox is linked to allow the use of an outgoing mail gateway. Details on how to do this can be found in official documentation Google.