Transaction email can scale up pretty fast and if you are using an ERP system, like ERPNext, it soon becomes the communication lifeline with your customers. Due to its high volume and "automated" nature of the email, most personal email providers like GMail do not like if you use their servers to send these emails and will soon start to block such requests.
Unlike your personal emails, transaction emails are generated from the ERPNext server and this means that services like GMail automatically do not trust it. Unfortunately many people use these services to send SPAM mail and the distrust of letting other servers send email is not misplaced. They make it hard for you to send such emails (like making to enable less secure apps).
Specialized Sending ServicesThe standard practice to send transaction email is to use a third party SMTP Relay service. Let me explain this in English. Emails are sent and received through separate systems. The sending system is called SMTP (Simple Mail Transfer Protocol) and emails received by either POP or IMAP. When dealing with transaction email, we are concerned with sending, so we need to find a service that will allow us to send emails, hence it is called an SMTP Relay service.
There are many such services you can sign up for and they have all kinds of pricing and generous free plans too. Some of them include:
How Does This Work?To send mail using any of these services, you have to sign-up and verify your domain. Once you do this, you can use their SMTP credentials in your ERPNext account and ERPNext will send those emails via your SMTP account.
These services will closely monitor your reputation and if you send mails that users will start marking as spam, they will shut down your service. Don't worry, they are not so harsh and if you are not sending a newsletter every other day, you will be fine!
ERPNext.com has Built-in Email ServiceIf you are using ERPNext on the erpnext.com cloud, don't worry we have this covered for you. If you do not setup any default outgoing, then ERPNext will send you emails using its own SMTP Relay (which is through SparkPost).
You can learn more about the plan here: https://erpnext.com/pricing
Hope this was useful and if you have any questions, please feel free to post them here.