Did you configure the SMTP option of php.ini to point to your postfix container (using the PHP_INI_SMTP environment variable?)

Hm, doesn't work for me. What about adding something like DMA to the image & then allowing to set the SMTP details through environment variables?

Edit: DMA doesn't even need to be configured to send email - the simplest way to set up sending email with it would be the following in a custom Dockerfile:

USER 0
ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y dma
USER 1000
ENV PHP_INI_SENDMAIL_PATH "/usr/sbin/sendmail -t -i [email protected]"

That being said, I'd really like this to support fully configuring DMA through environment variables, e.g. [email protected], DMA_CONF_SMARTHOST=smtp.postmarkapp.com DMA_CONF_STARTTLS=true, DMA_AUTH_USERNAME & DMA_AUTH_PASSWORD.

DMA_FROM would set the php.ini option, DMA_CONF_* would set the corresponding entry in dma.conf and DMA_AUTH_* would set username and password in auth.conf.

I've installed DMA & added documentation, but parsing the environment variables & generating the configuration is still to be done - I'll probably get that done in the next couple of days.

I try to make any enhancements in this repo (and first i have to clean PR). I'm sorry because you worked a lot on this PR but i'm not sure this docker image have to act like SMTP server. There is lot of docker image who do that.
Could you give me some use cases where the implementation of an smtp is required ?

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DMA doesn't really act like a full SMTP server - without anything, it just acts like the sendmail command usually does, so it sends the email directly to the recipient SMTP server. With configuration, it can be used to relay sendmail to a specified SMTP server (like your ISP, or Sendgrid, etc.).
Without any sendmail-like command installed, it's AFAIK basically impossible to use the mail() function with this image, which many PHP applications rely on. I'll try to get the PR up to date in the next days & will try if it finally works.