[eluser]jrlooney[/eluser]
Just got another clue.
It appears to be related to
bcc_batch_mode
&
bcc_batch_size
I was sending to 90 email addresses and getting the errors described, this was when bcc_batch_size was set to 20. I just changed it to 100 and all 90 messages went out fine.
Can someone from CodeIgniter please look into it and help me understand a fix, or let me know if it'll just have to be logged as a bug and taken care of in the next release?