@zarelit looking good. I just built your branch locally and ran it on NixOS stable, all good. Tested both the executable files provided and everything functioned as expected.

Do you have any interest in adding a module for this? A module would be pretty convenient for test boxes, plus a NixOS test would make confirmation the package is working automatic and easier for merging.

@aanderse Thank you for the review!
I actually use the derivation on non-nixos machines but I'm interested in writing tests and a module.
What use case do you have in mind for a services.mailcatcher module? Local testing?

Local testing primarily, but redirecting mail that all the test copy of production web sites on my nixops dev network generates, and then giving developers who use the network access to the mailcatcher web ui might be useful too.

If you end up writing a test and/or module please ping me as I'm interested.

So I started talking to the guys at work and realized this would have way more value than I initially thought. Pretty straight forward first stab at it, but here we go:

master...aanderse:mailcatcher

@GrahamcOfBorg test mailcatcher

ofBorg seems to apply this properly on the current master, I get the same error locally when rebasing your branch onto master (7e8d125 in my case):

machine# [   11.241912] mailcatcher[782]: Starting MailCatcher
machine: running command: nc -z localhost 1025
machine: exit status 1
(0.03 seconds)
machine# [   11.820546] mailcatcher[782]: ==> smtp://127.0.0.1:1025
machine# [   12.069383] mailcatcher[782]: /nix/store/f5nbyswzirqqiwgbq7f42slslyw1args-ruby2.5.5-thin-1.5.1/lib/ruby/gems/2.5.0/gems/thin-1.5.1/lib/thin/server.rb:104: warning: constant ::Fixnum is deprecated
machine# [   12.081162] mailcatcher[782]: ==> http://127.0.0.1:1080/
machine: running command: nc -z localhost 1025
machine# Connection to localhost 1025 port [tcp/blackjack] succeeded!
machine: exit status 0
(0.02 seconds)
(2.15 seconds)
machine: must succeed: echo "this is the body of the email" | mail -s "subject" root@example.org
machine# [   13.992932] mailcatcher[782]: ==> SMTP: Received message from '<root@machine>' (277 bytes)
machine# [   13.999045] sSMTP[802]: Sent mail for root@machine (221 Ok) uid=0 username=root outbytes=360
machine: exit status 0
(1.31 seconds)
machine: must succeed: curl http://localhost:1080/messages/1.json
machine: exit status 0
(0.10 seconds)
error: Died at (eval 19) line 6, <__ANONIO__> line 645.
(16.17 seconds)
Died at (eval 19) line 6, <__ANONIO__> line 645.
cleaning up
killing machine (pid 594)
(0.00 seconds)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
builder for '/nix/store/26fd8x881ih72vbkhsa1pj50z2cm88ag-vm-test-run-mailcatcher.drv' failed with exit code 255
error: build of '/nix/store/26fd8x881ih72vbkhsa1pj50z2cm88ag-vm-test-run-mailcatcher.drv' failed

Will investigate.

Also, I'd like to avoid having GCC in the mailcatcher closure but I think it happens at a lower level (e.g. in the bundlerApp code) and thus it might be good for another PR/issue

@Ma27 @aanderse thank you for working on getting this merged!

@zarelit I noticed mailcatcher 0.7.1 has been released. Can you please bump?
@Ma27 Incompatible change in the web server from 0.6 series to 0.7 series. Actual email message content no longer available under /messages/1.json, but instead /messages/1.source.

#58489

@GrahamcOfBorg test mailcatcher

@zarelit thanks!