Apache localhost mail() not being received with Mercury Server & Thunderbird

Question

I upgraded Windows XAMPP to 1.8.3 and am trying to get php working (again) on localhost with Apache 2.4.4, Mercury Mail 4.6, and Thunderbird client 24.1.1. Thunderbird can send and receive mail between two accounts. php's mail() is returning true but no mail is coming through.

php.ini is set to listen on port 25.

[mail function]
; XAMPP: Comment out this if you want to work with an SMTP Server like Mercury
 SMTP = localhost (I've also tried 127.0.0.1)
 smtp_port = 25

Any ideas what I might be missing? Thanks in advance.

Note: sendmail.ini also has

smtp_server=localhost
smtp_port=25

Answer 1

I commented out the following line and it worked.

; sendmail_path="C:\xampp\mailtodisk\mailtodisk.exe"

For good measure, I changed this to:

sendmail_path = "\"C:\xampp\sendmail\sendmail.exe\" -t"

I don't know why mailtodisk.exe is defaulted to being used, but it appears this simply writes the mail to a folder and it doesn't get delivered.