AS/400 QMSF stopped processing message. When sending large quantities of mail using Mail API.

We are in the process of sending emails to our customers but has come to a halt due to an MSF stopped processing messages as per example below.

We are running OS V5R3.

**SMTP distribution not sent.  Unable to parse data.   Message ID (TCP9676)        
**The requested heap space operation is invalid.        Message ID (MCH6902)
**The pointer parameter passed to free or realloc is not valid.    Message ID (C2M1212)
**Job 751654/QMSF/QMSF stopped processing MSF message.         Message ID (CPFAF98)

This was our scenario in a production environment, we had ended MSF and started it with 8 jobs with a status of *RESUME rather than a *CLEAR status. We are using the Mail API to send emails.
1.) First batch was to send out 5,000 emails, 3 MSF jobs failed and has a stopped processing message as per error above.
2.) 30mins later another 5,000 emails gets sent, and 1 MSF job failed.

My question would be why this is failing? Is there some sort of processing limit that we have reached? If so, what is that limit? Did emails from the first / second batch actually went out to the recipient? With 8 MSF jobs running, after it has stopped processing a certain amount of mail, will it continue processing the rest to another job or stopped completely?

We had tested a similar process in our development machine, with sending out 5,000 emails starting out with a clean MSF jobs and no other jobs were in progress at the time except our test. This was a successful attempt and received 5,000 emails.

Any help greatly appreciated.