Comment 3 for bug 1255611

As Dave said, this is indeed considered a feature :-) The fact that one or several emails are being sent is actually an implementation detail you should not care about, and is a consequence of the fact that each email is slightly personalized. The From/To headers are correctly set for each individual recipient (a single mail would force use to hide them completely or to incorrectly broadcast the emails of all followers, plus risk getting invalid "Reply all" effects).
And when the `portal` module is installed, this also permits adding a personalized link at the bottom of each email, letting the recipient either signup for a personal portal account using a unique invitation link connection to their existing "customer data", or directly open the corresponding record if they have previously signed up.

The fact that your mail server throttles emails is a configuration detail in your installation, and it's definitely recommended to fix that because OpenERP is not a true MTA/MUA: it expects a working SMTP server at all times. If you want that local SMTP server to be in charge of queuing the mails to comply with a maximum outgoing email rate, that's up to you.

Note that the outgoing messages are queued on the OpenERP side in the mail.mail model, but OpenERP does not currently distinguish a 4xx temporary failure from a 5xx permanent one, so it will mark them as permanently failed in all cases. There's a retry button if you go to the list of failed deliveries (See /Settings/Technical/Email/Emails), but that requires a manual action for each message, otherwise the queue runner will never retry them.

As a workaround for your particular situation you might want to add a scheduled job inside OpenERP to force the retry of all failed deliveries automatically.

Hope this helps,