Automatize mail answer - without Auto Answer headers

Hi,


I'm looking for a mail provider that could help me to automatize answers for each mail receveived on a specific address.

I need to be able to send pre written mails like an auto answer for Out Of Office but without putting Auto-Answer headers !!! Mail receiver (both SMTP server & user) need to understand the mail as a normal one.

Do you know what service could I use to do so?
I already use MailGun, pretty good; but I can't send auto answers without writting a bit of code. I was wondering if a SMTP provider already hooked auto answers.


Thank you !

By the way, after few tests trying to automatize MailGun replies I actually noticed my mail were never handled by the final SMTP server (my guess is that they are blocking mail from famous developper mailer (MailGun, SendGrid and so on). They know mails are comming from MailGun as MailGun is adding few headers specific to them (like X-MailGun-ID).... and there is no way to disable those headers

So, my last option is to look for classic mail service - with much more low rate in terms of mail/day. Then I need to scale this with a high number of accounts.

Do you have any mailbox in mind that you know is having pretty high number of mail to be sent per day ?

Note: I also need to check if all those developper mailers are using the same header technic making them easily identifiable (probably to avoid loosing reputation but it is worth trying)


Thanks,