mail() truncates message on full stop between two newlines

Description

The following code:

<?php
mail("myself@mydomain.com", "Subject whatever", "Foo bar\n\nWhatever\n\n.\n\nLorem ipsum dolor sit amet");

Resulted in this email body being sent:

Foo bar

Whatever

(not sure whether with or without the \ns at the end and how many of them)

But I expected this output instead:

Foo bar

Whatever

.

Lorem ipsum dolor sit amet

I know that a line with only a full stop is how the email body is terminated in the SMTP protocol, but this is not the SMTP protocol, this is a php function that takes a message and sends it via email. If the caller is supposed to encode the message the way it would be encoded in the SMTP protocol, the docs should say so; but again, that would be stupid.

PHP Version

8.0.12

Operating System

debian 10

首先定位 PHP 的 mail() 实现，以及为本地邮件传输序列化邮件正文的路径。然后复现提供的 PHP 8.0.12 示例，再添加或更新回归测试，以确保仅包含一个句点的行能够被保留，并发送完整的预期正文。