Please see the following link for further information: https://docs.microsoft.com/en-gb/microsoft-365/security/office-365-security/atp-safe-attachments?view=o365-worldwide
Equitas Trust use ATP (Advanced Threat Protection) to scan links and attachments that are sent over email.
"Safe Attachments in Microsoft Defender for Office 365 provides an additional layer of protection for email attachments that have already been scanned by anti-malware protection in Exchange Online Protection (EOP). Specifically, Safe Attachments uses a virtual environment to check attachments in email messages before they're delivered to recipients (a process known as detonation)."
When you revive an email with an attachment you will see a message like the below. The attachments will be being scanned by ATP. Once the scan has complete the message will show as unread in your inbox and will allow you to open the attachments. You will not be able to open any attachment whilst the ASTP scan is in progress.
Depending on the size of the attachment/ number of attachments this can take anywhere from a few seconds to a few minutes.
If you require further support please email: firstname.lastname@example.org