I am running a web service that currently sends confirmation emails out to new users via the gmail smtp servers. As I'm only getting a few new users each day, this hasn't been a problem.

I've recently added new features to the webapp that will require a customized message to be sent out to each user every day. Think of this as similar to the regular messages LinkedIn sends out that give you a status report on the activity in your network. Every user's message will be different. With thousands of users, this means thousands of unique messages will be sent each day.

Edit: I've since found that these types of email are called "transactional or relationship messages". Spamtacular has a good article on differentiating between marketing and transactional email.

I don't think using gmail's smtp servers will cut it anymore, but I don't know that for sure. I don't know what gmail's maximum outgoing messages per account is (it might be 100/day), but they limit outgoing mail to 500 recipients per message. I'm not sending a single message to 500 recipients, but I'm going to be sending 1000's of customized messages with each recipient getting one per day.

I'm interested to learn any best practices for doing this (especially for Java-based webapps). Here are some of my thoughts and concerns on it:

• Should I set up my own outgoing mail server? If I do this, it seems like I'll have all sorts of other issues to worry about, such as preventing mail server abuse, monitoring bounces, allowing ways to opt-out of emails, etc. Are there any tools or services to help with this? Maybe something like OpenEMM or a services like MailChimp? But those seem focused more toward email marketing campaigns.
• I don't think I should have the webapp itself handle sending emails as it currently is for new user signups. I'm thinking I should setup a separate messaging server that can access the same backend/datastore as the webapp. Thoughts on this?
• Should I consider setting up some sort of message queueing service to help with this, such as JMS, RabbitMQ, ActiveMQ, etc.?
• Do I need to provide users a way to opt-out? Do I need to flag these as bulk messages? I don't really consider these email marketing messages, but I'm unsure what is considered appropriate or proper netiquette.

Any advice is appreciated. I'm also very interested in open source tools or web services that simplify things and could help me to ramp up as quickly as possible.

Thanks!