look in ow base mail  in database and see what the emails are 
from there u be able tell whats sending them base or a plugin etc

and turn ur cron off to stop the mails untill u get it fixed so u not get put on to many spam lists

it could be someone abuse email on contact inporter 

its just the regular notifications repeating over and over again  

if it was me id delete that member 
flush the mail db  
do like 10 test join process n see if it happens again 
if not then email that person say sorry n they need rejoin 

if happens again on test join or on somene else join then start deactivate plugins untill find the problem

and get hosting to flush your mail cue 
as there will be heaps in the cue to be sent 

its going through all the notifications and repeat sending all users email

delete ur cron and get hosting to flush ur mail cue 

then try work the problem out 
by deavtivate all 3rd party plugins flush the mail bd and see if keeps happen

the only change i made was after the update I added memcached to the 0x speed optimizer.

I also enabled the Database query caching lifetame.

For now i disabled the Database query caching lifetime and im going to try it out to see. I had issues with that in the past I thought it might have been fixed in the latest update.

i flushed the mail que

does the mail que in the database ow_base_mail  auto delete when its done sending?

I just checked it again and now the table is empty

yes if the mail db is empty they been sent 

so u flushed the mail db what about the server mail cue it will be full mails to be sent 

fulsh mail cue needs be done in whm or ssh 
if u not have u need ask hosting to flush it 

I flushed it also it already had 450 qued before i stopped the cron. I use webmin.

It seems like it has stopped now that I disabled the  Database query caching lifetime feature on the 0x speed optimizer plugin

i have enabled the Database query caching lifetame.
and dont have mail problems

webmail will not flush the mail cue 

i can't think of anything else that might have caused it. 

i dont have time to keep telling u i have heaps my own problems to sort 

No i use webmin it is a web based gui admin tool for linux servers. It let me flush the que from my post fix server