How to send multiple mails at once with - email

I am interested how mailgun or similar services does send same message to multiple users with one call to user list, like described here
Is it just simple loop for each user or there is bulk mailing options for all SMTP servers.
I just want to create my own server for marketing newsletter to our users and was thinking about how to organize sending of mails.

I cannot comment on the back end of how specifically mailgun works, but I know when I have needed to do it in the past I have done it with a message, php array of emails and just sent them using php send-mail, just a for-each to loop through the list of emails and they would all be sent right off.

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Detecting if emails send by our application is marked as spam

We are developing an application that will send periodic updates and notifications to users as email. The user can opt-in and opt-out of this service via a subscribe option. However we are finding that some users are making the email as spam and as a result our account is getting suspended... Is there any way to track if our emails has been marked as Spam by a user, so that we can stop sending emails to them...
We have a GSuite service and are using Gmail SMTP to send emails
No.
But you can check if your domain is on any blacklist with tools like mxtoolbox.com. And contact those blacklists with the question what you could do to be removed from the list.
In order for a mail to be classified as spam it has to fail a multitude of tests maybe your mails have specific words in the title or the senders address is way to weird or the header is getting corrupted in a certain way or and maybe that's your problem: many people are custom filtering your emails as junk/spam.

Can I post a new topic on a google group using Go?

I'm writing a program that should send emails to multiple users with content extracted from an excel spreadsheet. I know how to do this using the net/smtp package in Go, but I would like to know if it's possible to send an email with the sender being a google group (i.e googlegroup#gmail.com) instead of my email without resorting to using the gmail API? Currently I have a working program that can log in through an email and password, which is then used for auth credentials, but seeing as that google groups don't have the same kind of interface I'm not quite sure how to change it so emails are sent from a group instead of an individual user.
Each google group should have an email address associated with it. golang-nuts is golang-nuts#googlegroups.com for example. Any mail sent to that should be posted to the group, assuming it is from a member of the group.
In order to send from your own gmail account, you can use gmail's outgoing smtp feature with the net/smtp package. Configuration is explained better on this digital ocean post

Email Intermediary Between Sender and Receiver

I am using SES for emailing (currently only sending but I'm open to using it for receiving as well), EC2 server for web app and RDS.
Web application written in Java.
I would like to act as intermediary for users that email each other through my website by having all emails sent between users go through me.
The purpose is to conceal the emails of the two users who are emailing each other.
I thought of doing something like this:
1) The initial email is sent on the website, there it receives a unique ID which is stored in the database (containing the email addresses of the sender and receiver).
2) All subsequent emails between the two users are sent to the website's email address with the unique ID appended as a label (eg:bob+[uniqueId]#domain.com).
3) The email is accordingly routed back and forth between receiver and sender (I perform minor modifications to the email).
(Airbnb does something similar when users of its site message each other).
An extra caveat is that I would like attachments to be able to be included in the emails as well (and thus, they would need to be forwarded).
Is this the correct way to implement this functionality?
Should I do it differently?
If this is the correct approach, any references for how to get started? Specifically, I'm not sure how to use a lambda function (if that is what I would be using) to send an email, or to query my RDS.
Thank you
This sounds similar to private email systems I've seen on other sites. I think you have the correct idea. I would recommend using SES for both sending and receiving, and use a Lambda function to process the incoming emails.
You can have SES fire a Lambda function when you receive an email: http://docs.aws.amazon.com/ses/latest/DeveloperGuide/receiving-email-action-lambda-example-functions.html
The other option for triggering processing of incoming emails would be SES->SNS->Java application webhook. Or you could queue them up via SES->SNS->SQS and have your Java application subscribe to the SQS queue.

Custom mailing list system

On our website we use a MySQL table which contains all users. Some of the users should be able to send mails to all users. Unfortunately I haven't got the right idea how to implement such a system.
My current design:
A privileged user writes a mail to users#website.com using his personal mail software.
A server is waiting for incoming mails.
As soon as it receives a mail, it matches the 'From' field with the database.
If the user is not found or hasn't got the special flag, discard the mail.
Forward the mail to all users (mail addresses are saved in the database)
Send the sender a confirmation mail.
How can I implement such a system? I'm not required to stick to a certain programming language even though C++, PHP or Node.JS would be preferred.
As a jumping off point, look into IMAP/SMTP. You can, with any language, log into an email inbox, get emails, read them, etc etc etc. Set up your mailbox with an account users#website.com that receives legitimate emails. Then set up a cron job that checks the inbox every 5 minutes or so. If no emails are found, then you are done. If there is an email there, process it, run your checks against your DB, and then send it using SMTP, and then send the confirmation to the user.
If handling IMAP is too hard, you may just want to make a form on your website that privileged users can use which sends the emails to the users.

Best way to send a bunch of emails without user input on iOS?

First, let me get this out of the way and say this isn't a spam application. I'm writing an app that allows users to share files with multiple friends easily via email.
Right now, I'm using SKPSMTPMessage to send out emails to a list of recipients at the user's request. It works, but I wonder if there isn't a better solution here. For example, if it might be better to send the smtp info and attachments to my server, and conduct the sending from there in order to avoid repetitive attachment uploads on the device.
I am also considering how to handle cases where there is no connection available (determining connection error vs other kinds of errors and persisting the failed message for a later retry).
Any advice on the best way to structure this service would be greatly appreciated.
In order to customize the user experience for sending e-mail you need access to their email details, IMAP, POP, etc. if you want that user to be the one sending the e-mail.
The solution I use is to have the from address be an e-mail I control and use a 3rd party email provider, ex: SendGrid, PostMark, Mandrill and plug in to their api's to send e-mails.