What the message looks like
450-4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered450 4.2.1 Recipient address rejected: User over quota
What it means
The recipient's mailbox can't take more mail right now, so the receiving server is asking the sender to try again later. The standard name for the permanent version of this status is "mailbox disabled", but the 4 at the start makes this one temporary.
The most common version comes from Gmail, when a single inbox is receiving more email than Gmail will accept in a short time. If it keeps happening, Gmail can switch to a permanent 550 5.2.1 with the same wording. Microsoft uses 5.2.121 and 5.2.122 for its hourly receiving limits.
If you're the recipient
Usually nothing. The sender's server keeps retrying, and the email arrives once the rate drops. If you see this often, something may be sending a large volume of email to your address, such as a mailing list, an automated alert or a forwarding loop.
If you're the sender
Let your server keep retrying. If you're sending many emails to the same person in a short time, spread them out or combine them.
If you use addy.io
addy.io sends 450 4.2.1 Recipient address rejected: User over quota when too many new aliases are created on the fly within an hour. The emails are deferred, not rejected, so they arrive once you're back under the hourly limit. See Is there a limit to how many aliases I can create per hour?
If Gmail sends this error when addy.io forwards to you, addy.io retries automatically.