This is down to the sheer size of our mail platform and large quantities of mail we send to these various providers. We have been working over the past few months on improving the reputation of our platform by introducing new filtering technologies to help stop the emails which can cause negative marks left on our systems.
While this situation has improved, we are aware that from time to time users will see bouncebacks from the above providers. Our advice for these cases would be:
Add an SPF record:
If you're using our servers alone, the following SPF record can be added to your domain names DNS configuration as TXT record:
Use an alternate mail server:
We have a lot of different mail servers, all with different IP's. If one mail server can't send your email to a certain provider, try sending it via another. Our support teams can help identify a suitable server for you.
Alternatively, we have a webmail interface which uses a more sophisticated filtering system.