allow external certifcates and smtp for other services

[FarisZR]

hi,

i would like to use this image but two things are stopping me

• allow smtp usage for other services since if you use this image you can't have any other mail service running at the same time, this image needs the ability for other apps to use it as mail server through smtp for example

• external certificate support i have another app managing my certificates so no need for it to be regenerated again, however i can make it work

[nguyenkims]

Hey if you also use Postfix for other services, I think the current way of running Postfix outside of Docker is more flexible and easier to tweak.

About the "external certificate support", that's indeed a good idea!

[LordChunk]

My fork already has support for external certificates. I'll go work on a PR that works with this fork.