Each UNIX vendor, it seems, rewrote or heavily customized sendmail. This has lead to sometimes conflicting implementations.
Case in point: -tNormally, you invoke sendmail(8) with a series of arguments indicating the subject of a message, the recipients, etc. When invoked this way, the command expects a message on standard input, waits for EOF, and then sends your message along.
However, sometimes you don't want to have to fiddle with command-line parameters; you've already written a perfectly fine message with headers.
-t is generally passed to sendmail when you want to build a message envelope from an already-formatted message, with headers, etc. For example, if you had a file foo.txt with a body like this:
From: Luke Faraone
To: John Smith Subject: Hello, world! Hi there.
you could send the message with a simple invocation of cat foo.txt | sendmail -t. The system would take care of ensuring a Message-id was appended if appropriate, and queue the message to be sent. However, it is when you do slightly more complex invocations of sendmail that things get ambiguous.
It turns out that implementations differ on what exactly it means when you use -t in combination with naming destination addresses after the arguments to sendmail. exim4's documentation describes the situation in greater detail:
Thus, there's basically no mechanism for a program to know which behaviour to expect. God forbid two programs are installed on a system that expect different behaviours!
It appears that the default behaviour of Ruby is the opposite of what exim4 (Debian's default mail client) expects. This has resulted in numerous bug reports. Some replies suggest changing exim4's defaults, while others advocate overriding ActionMailer and friends to use sendmail -i instead, without -t.
That said, its not really clear who's wrong here; at no point does there appear to have been a definitive specification for sendmail, and as such we can hope for defined behaviour by common custom at best, and a sea of incompatibility bugs at worst. Amusingly, POSIX standards have nothing to say on this subject of sendmail at all; it defines that a mailx command must exist, but says that its sending mode may be implementation-specific.
As Matthew Garrett writes, there's not enough gin in the world.