In order to develope my own subscription module, what is the best practice sending mail to a list of users: send only one mail with multiple bcc recipients or send one mail per recipient? In case of send one mail per recipient, should I delay each message? how long?
I wouldn't like my ip was blacklisted.
I suppose it all depends on how many recipients you will have.
I'm not aware of all of them, but there are probs and cons for both.
Sending an email to each individual recipient will allow you to personalise the subject/body, use tracking URLs etc. That said, it will take longer for PHP to send all the emails, depending on how many recipients you have. For example, if you have 250k recipients, you may be waiting a while. In addition to this, it may also put a strain on your server.
bcc will take a lot less time as the process will only happen once, but it won't allow personalisation, tracking URLs and you may run into a bcc limit.
Your best bet will be to utilise a 3rd party SMTP email distributor which will take the heavy load off your hands. Mandrill is one of them but I don't believe they have a free package anymore so you may want to look around for others.
Note that this is not exactly a Joomla question.
You also have provided only a few details, and also it's not clear of what exactly you are after. And what is the subject of the best practices you ask? Best practices regarding avoiding your IP being blacklisted? If there is a such possibility then I guess the chances are the same.
Avoiding server IP from blacklisting on spam filters is certainly not a Joomla topic, while being a broad one with many factors to be considered and a list of items to be configured on the server level. It is also related to the way a user is using the email service. If you start sending spam - or your receivers consider it as spam, then most likely you will be blacklisted.
Generally, sending mass emails today is something that has been moved away from the same website hosting server and usually handled by related email marketing services, like Mailchimp or ConstantContact etc.
Check also this older question in this site: Mass mail 50k + users, solution?
