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A mail forwarding service provides a mailing address (usually in your domicile state) and collects your mail. You let the forwarder know where you are (or will be) and they bundle your mail and send it to you. Some services will scan mail for viewing on the web, watch for critical pieces of mail to come in, etc.
If you intend to use your mail forwarding service address as your domicile address take note of the county where the forwarded is located. There are reports that many mail forwarding service addresses have been blacklisted and may not suffice as a residence or physical address.
There are also ad hoc ways to receive packages.
If you are using the forwarder for residency remember to check any county restrictions.
The United States Postal Service runs a forwarding service called Premium Mail Forwarding Service. It is limited in usefulness, as it is very expensive and can be active for only a year.