Question:
Does your software create a different email for each recipient or is this a function of the email provider (Yahoo Business)?
For example an email with attachment is sent to customer@abc123blahblah.com and BCC to sales@abc123blahblah.com (sames as send from) and BCC to it@abc123blahblah.com.
The problem is that about 1 out of 5 emails created the attachment(PDF of invoice) or body text missing from some of the emails.
The answer to this question is found by reading this Chilkat blog post: http://www.cknotes.com/?p=40
You'll see that in the SMTP client-to-server communications, the MIME of the email is only passed once regardless of the number of recipients.