Problem with delayed emails distribution

After a bit of hand coding and readings on this forum, I was able to have my form successfully sending mails to my distribution list, with unique recipient data for unsubscribe, progress bar working, etc. That was with a distribution list of 5 addresses...

When I raised it to 385 addresses, I kept running into vague 500 errors. With the buffer I have to keep for other forms also using SMTP resources, I can only send 200 per 24 hours. With this 200/24h limit, I tried different time schemes for distribution in between these 2 :
$BurstSize = 200; $BurstTime = 1440; $WaitTime = 1;
$BurstSize = 0; $BurstTime = 0; $WaitTime = 432;
In the 1st case 200 emails were sent, even if the last console event (from the progress bar) before the 500 error states it was processing email 131 of 385...(?!?).
In the 2nd case only the first email was sent before the 500 error.
Makes me believe my server somehow doesn't like idling for too long on a php page...
Then I remembered someone mentioning on this forum that closing browser window should not be a problem, and that mail distribution should continue... Am I stupid enough to try that ? Yes. Well too nice to be true, only 1 email was ever sent...

What else should I try ?!?

I'm afraid you'll tell me the solution to my problem resides in some server settings I won't be able to access. I'm on Linux shared hosting at Anonymous... euh... I mean GoDaddy.

Any hint appreciated.

Try increasing the Burst Time and decreasing the Burst size.

200 emails per 24 hours is your maximum, riding directly at the maximum is prone to failure. just try doing that with your car engine, keep the engine revs at the red line and watch the engine break down very quickly.

Instead throttle back to be bellow the maximum, try 175 or 150 emails every 26 hours.

Bear in mind that if you are using GoDaddy shared hosting, you are using a shared SMTP server.

Thanks Jason.

I did what you suggested. The 150 first emails are sent, then 500 error again.

The SMTP server is indeed shared. That sure impacts performance in delivery speed, but not the daily SMTP relays limit for that particular domain, which is 250. 200 is what I decided to keep a few relays for other forms (no one else is using the site at the moment, so I didn't reach that limit through the other forms...)

The error does not occur while sending, but when it should be «paused».
On every tests I did, whatever Burst size & time and Wait values I specified, the number of emails I specified for the first batch (may it be 1, 10 or 200) was always sent successfully. Depending on the values I entered, the process ran successfully for periods varying between 1-2 minutes to over 40 minutes. It is 4-5 minutes after entering what should be «paused status» (while not sending anything to respect limits) that the browser pop the 500 error page.

I'm pretty sure the 500 error comes more from the idling php page rather than reaching the SMTP limit. But that said, even after turning on all php error loggings possible and adding the 3-4 php lines you suggested in other threads, GoDaddy's Apache and Error logs are still completely useless.

What is the way to process the php file containing the UE behaviour, start the sending process, and redirect user to a confirm page ?

I have created a support ticket so we can look into this issue further.

To view and edit your support ticket, please log into your support history:
supporthistory.php

If anyone else is experiencing this same issue, please append to this thread.