SMTP Error (451): Failed to add recipient "email" (Temporary local problem - please try later).

Here are the steps to follow:

1. Run /scripts/mailperm --force, then run /scripts/updateuserdomains.

2. Optionally, you can run /scripts/forcelocaldomain domain.com (this will add the domain.com to the /etc/localdomains file).

3. Alternatively, manually add the domain to the /etc/localdomains file.

Note: Ensure that the domain is listed in the /etc/localdomains file.