Strategies for sending weekly newsletter emails from Rails - ruby-on-rails

Question

So we have about 50,000 users who have signed up for a weekly newsletter. The contents of this email is personalized for each user though, it's not a mass email.
We are using Rails 4 and Mandrill.
Right now we're taking about 12 hours every time we want to fire off this emails.rake task and I'm looking for a way to distribute that time or make it shorter.
What are some techniques I can use to improve this time that is only growing longer the more people sign up?
I was thinking of using mandrill templates, and just sending the json object to mandrill and have them send the email from their end, but I'm not really sure if this is even going to help improve speeds.
At the 50,000+ level: How do I keep email sending times manageable?

Answer 1

Looks like you could use MailyHerald. It is a Rails gem for managing application emails. It sends personalized emails in the background using Sidekiq worker threads which should help you out in terms of performance.
MailyHerald has a nice Web UI and works with email services like Amazon SES or Mandrill.

Answer 2

You need to probably look into Merge Tags on Mandrill. It allows you to define custom content per email. So you can break your newsletter sending into fewer API calls to Mandrill instead of 1 per email. The number of calls will just depend on the size of your data since I am sure there is probably a limit.
You can just create a template and put in merge vars such as *|custom_content_placeholder|** wherever you need user specific content to be placed. You can do this templating in your system and just pass it into the message or you can set it up in Mandrill and make a call to that template.
When you make the Mandrill API call to send an email or email template you just attach the JSON data such as:
"message": {
"global_merge_vars": [
{
"name": "global_placeholder",
"content": "Content to replace for all emails"
}
],
"merge_vars": [
{
"rcpt": "user#domain.com",
"vars": [
{
"name": "custom_content_placeholder",
"content": "User specific content"
},
{
"name": "custom_content_placeholder2",
"content": "More user specific content"
}
]
},
{
"rcpt": "user2#domain.com",
"vars": [
{
"name": "custom_content_placeholder",
"content": "User2 specific content"
},
{
"name": "custom_content_placeholder2",
"content": "More user2 specific content"
}
]
}
],
You can find more info on Merge Tags here:
https://mandrill.zendesk.com/hc/en-us/articles/205582487-How-to-Use-Merge-Tags-to-Add-Dynamic-Content
If you are familiar with handlebars for templating, Mandrill now supports it with the merge tags:
http://blog.mandrill.com/handlebars-for-templates-and-dynamic-content.html