Adding CUST Number to Email Subject Line

Hello, all—

We have a workflow-based system that automatically sends statements to our customers at the beginning of each month. The emails are based off a template, which the workflow uses to deliver those statements.

However, we've been noticing that every now and then the emails aren't delivered (this happens when multiple customers use the same invoicing email address), likely because the subject line is the same for all the emails so the mail servers are blocking those emails due to suspected spammy behavior.

So what we'd like to do is to add the CUST-#### to the each subject line, as that will make each subject line unique and thereby avoid the emails as being flagged as spam.