Fatal error: Out of memory - CiviMail
If you are processing more records at the same time - you will run out of memory - 

In order to prevent Out of Memory the best way is to process less records in a single go - civimail does process records in a batch (like 200 approx.) at a single go 

Whenever we create PDF in bulk, mail in bulk always we use to end up in Out of memory even in local PC or local server - so processing min amount of records should fix the problem 

I guess the main problem for this Out of memory Issue is:
Civicrm DAO uses more memory and once the process is finished it doesn't free the memory at all. So php memory keeps on building and eventually - so we end up in Out of memory 

I guess this helps!!!!