Sorry the problem is different from the title. Now we found all the emails were actually sent but all sent to spam folders!
We use one gmail account to send all emails. We set the gmail account as the the store contact email address as well.
the email setting is
1. Use PHP's mail() function (recommended; works in most cases)
not SMTP
We also notice that all activities which need to send emails become extremely slow, for example,
1. registration
2. confirm order
3. resend email (BO)
4. send comments(BO)
I tried setting _PS_DEBUG_PROFILING_', to true and attached the results here
It seems no problem with the time of database access and file loading. The cpu loading is also okay.
Can any one explain to me what is the next step I should try?