The most common causes for high volumes of queued messages is whitelisting or firewalling of our delivery IP’s:

1. Whitelisting IPs on your Firewall. Your firewall rules do not include the EU servers -
Outlined in 2016 blog post -

2. Whitelisting IPs on the server. If your mail server supports whitelisting please update with the same IP’s as listed above.  This is a different issue than firewall rules, as it is usually a rate limit or blacklist / whitelist on your mail server itself.
Keywords: Greylisting, Whitelist, Throttle, Limit

3. Spam / Virus Scanning. Your mail server may have Spam / Virus filtering configured locally in addition to the Spam / Virus filtering that we provide. If so, please whitelist our sending IP’s.

4. Your mail server is down. In this case please ignore this message (if you have already made sure that our IP’s are in your firewall / whitelist) And we will continue to queue your messages until your server comes back online or 10 days.