That will be
Official page: http://untroubled.org/nullmailer/
Official git: https://github.com/bruceg/nullmailer
And here is some guides:
General nullmailer setup, Nullmailer + FakeSMTP server
It still runs as service though, since it has queuing in there.
If you want a completely serviceless MTA, please have a look at http://msmtp.sourceforge.net - it doesn't require to run an MTA service, instead it works as a relay, by sending your mail via an external server. You can combine it with FakeSMTP script to get what you need. Beware, that relaying mail via an external server has a huge downside - your PHP script/web-server won't respond until the mail is actually send. Most public mail servers implement a delay for authorization, which can be as long as 30-40 seconds. So you'd have that delay for each & every letter you send (for a user it will look like script is not responding).
All those apps above are really sending mails, and can be used in production environment. If you want a developer version which does almost nothing - please have a look at this shell script: https://github.com/Sanchiz/fake_sendmail.sh, you can easily modify those 20 lines to your needs (first of all remove that line that sets 777 on resulting file, you don't want that), though I'd recommend to go with something less simple...