Intermittent Internal Server Error

My client is sending out a number of 'bulk emails' to around 100 people and is getting a frequent, (but intermittent) error message, so some mailings go out OK, others do not...

  Internal Server Error

The server encountered an internal error or misconfiguration and was
unable to complete your request.

Please contact the server administrator,  

The site is hosted by another party, and I cannot find an error log.

Will changing things like timeout or email frequency help to this or is it a hosting settings issue ?

without a more informative error message, it is impossible to tell what the cause of the error is.

I would contact the host to ask then for access to the error logs to determine what is really going wrong.

changing timeout or email frequency is really just a wild goose chase at this stage, you can certainly try it, but I cant say if it is those settings or a hosting settings issue at this point, there is not enough information to say one way or another.

I moved the site to a test subdomain on my own host.

After pressing send, the progress bar gets to around 73/74 of 100 before I get the error message

  Network Error (tcp_error)

A communication error occurred: "" The Web Server may be down, too busy, or experiencing other problems preventing it from responding to requests. You may wish to try again at a later time.
For assistance, contact your network support team.

  

I tried changing the burst size and time, but it seems to make no difference.

Update

I changed the wait time to 0.25 seconds ( can it go any lower ?) and it now processes all of the emails.

Is the problem due to server timeout ?

moved to another thread