Workmail being bounced by Mimecast customers - is there a way to solve this?

As I understand, you're managing two WorkMail organizations, and emails sent from both are currently being bounced when the recipients are using Mimecast. This appears to have started recently.

Since the emails are being routed through shared IPs from SES, this behavior typically occurs when Mimecast applies strict filtering policies against such traffic. In this case, the best course of action is to provide Mimecast with the predefined list of Amazon SES IP address ranges, specific to the region you're sending from. These IP ranges remain consistent and are the recommended method for allowlisting SES traffic.

Unfortunately, AWS cannot contact Mimecast on your behalf, as it is a third-party service and subject to certain privacy policies and access restrictions. For this reason, it's necessary for the Mimecast recipient or account owner to reach out to them directly to request IP allowlisting.

You can find the region-specific SES IP ranges in the AWS documentation on IP address ranges.

[+] IP ranges used by Mail Manager SMTP relays: https://docs.aws.amazon.com/general/latest/gr/ses.html#ses_mm_relay_ip_ranges

Hi,

I've pinged the service team to take a look at this. Something else you can do is verify your domain configuration, I can see that not all of the required SPF records are correctly set for your organizations domains. This will also contribute to a poor deliverability for your domain. Please take a look at the domain page for your organization to verify all required records are correct.

Kind regards, Robin