The 5 email per second has me concerned. Two scenarios:
1) A thread that is heavily subscribed to gets a reply. XF sends out notices to 50 subscribers of new post. If sending all at once will breach the 5 emails per second.
2) Newsletter mailing to all members. No delay will likely blow past 5 emails per second.
1) Does anyone have a higher send rate than 5 per second?
2) How many emails is XF likely to attempt to send per second all out speed?
3) Is there any simple way to "throttle" the send?
4) I think I read that Amazon SES dumps any emails exceeding the send rate instead of queuing them. Stupid IMHO but if this is the case no choice but to increase send rate per second or devise a send throttle mod