That error code translates as "too many messages sent".
In order to prevent spam, your email provider's server is configured to throw a wobbly (and generate that message) if you try to send out loads and loads of emails within a short period of time. As you could be using lots of different email addresses for your (theoretical) spamming activities, the server identifies you by your IP address, rather than by your email one.
However (through using a VPN) you've chosen to hide your real IP address, meaning that the email server sees the IP address of the VPN instead. Thousands of other people will also appear to have the same IP address so, if lots of you all try to send mail at once, the email server sees it as an attempt at mass spamming. (Also, if someone really is trying to send out spam through the VPN, at the same time as you're posting, your email will be regarded as part of that spam because there's no way that the server can see the difference).
The only way around is to try a different VPN address or, far simpler, to disable your VPN when sending out mail.