Sinch has to ensure a high volume of SPAM’s does not impact our IP reputation. Reputation (or sender scores) have a direct co-relation on deliverability. Also, complaints due to SPAM can result in temporary or permanent blacklisting.
One approach to overcome SPAM is to slow or throttle traffic to be delivered in specific time windows only and check for deliverability. The other is to ensure emails sending domains (or email addresses) are marked as safe senders by recipients.
High sending scores do not necessarily ensure that emails are delivered to an inbox. SPAM filter rules are always evolving and an email could be delivered as SPAM for any one of the following reasons: