Bulk Email CRM app

@Franois Van Lerberghe : please describe in more detail what you want to do. SendGrid and the like set up everything you need. You can send out 100k mails per month on the cheapest plan for SendGrid. If you need something in addition you need to talk to the companies.

I want to develop similar feature to the orignal poster request. Let me explain again :
I want to develop a feature in my application in order to allow the end-user send by email invoices or some sort of notifications (= transactional emails) to his customers. The amount of emails sent will be 100 per day or 500 once per week for example.
Some end-user have domain name and web hosting but some doesn’t have one.
If I use SendGrid, the end-user must have a domain name and web hosting and it must be configured to use the service (DKIM, SPF, …). If I have 2 end-users, I can configure it manually for each one. But If I have more than 100 end-users and some that haven’t any web hosting, that’s not really a workable solution. In this case, I must find another solution. Am I wrong ?

Your app would use your credentials as default. If your users have IT knowledge then let them configure the email stuff themselves.

[quote=439498:@FranoisVanLerberghe]But If I have more than 100 end-users and some that haven’t any web hosting, that’s not really a workable solution. In this case, I must find another solution. Am I wrong ?

[/quote]
I think registering a domain name with a cheap domain name service is a workable solution. This solution costs only a few dollars each year and gives you a unique domain for each of your accounts. You don’t have to deal with customers without their passwords for their web site (or without a web site altogether) and you can create an email forward to their true email if one of their clients replies to the email. This way it segments your clients into different DKIMs.

If you use a single DKIM for all your clients, you could set up a different return email address for each account (like client1@myinvoice.com, client2@myinvoice.com) which would forward to their correct email address should the customer reply to an email. This also solves the DKIM issue.

Interesting suggestion. I’m going to investigate this way.
Although I don’t like to have a single DKIM for all my clients nor use my own credentials. Because if an user is doing bad things causing the blacklist of my system, all clients (and me) are concerned. But perhaps, as I can manage internally the sending, I can minimise the risk.
Thank you.