For an application I'm testing I'd like to create an autouse=True fixture which monkeypatches smtplib.SMTP.connect to fail tests if they try to send an email unexpectedly.
However, in cases where I do expect tests to send emails, I want to use a different fixture logging those emails instead (most likely by using the smtpserver fixture from pytest-localserver and monkeypatching the connect method to use the host/port returned by that fixture)
Of course that can only work if the autouse fixture is executed before the other fixture (loaded as funcarg). Is there any specific order in which fixtures are executed and/or is there a way to guarantee the execution order?
IIRC you can rely on higher scoped fixtures to be executed first.  So if you created a session scoped autouse fixture to monkeypatch smtplib.SMTP.connect then you could create a function-scoped fixture which undoes this monkeypatching for one test, restoring it afterwards.  I assume the easiest way to do this is create your own smtpserver fixture which depends on both the disallow_smtp fixture as well as the smtpserver fixture from pytest-localserver and then handles all setup and teardown required to make these two work together.
This is vaguely how pytest-django handles it's database access btw, you could try and look at the code there but it is far from a simple example and has many of it's own weird things.
In which order are pytest fixtures executed?