CodeIgniter SMTP 电子邮件 - 用等号替换的字符

我正在使用CodeIgniter电子邮件库使用我们的Exchange服务器发送电子邮件。我遇到的问题是电子邮件的内容被弄乱了。

有一些单词被等号“=”替换，我尝试了2个不同的Exchange服务器（它们位于不同的位置，没有任何关系），我仍然遇到同样的问题。如果我使用任何其他服务器作为SMTP服务器发送电子邮件，一切正常，内容保持不变。

发送前的内容：

Dear Customer

Please find attached a comprehensive explanation of how to get our brochure of Angola. This has been sent to you at the request of Alex.

The information has been taken from www.example.co.uk  "Company name" is one of the leading tile and marble companies in the UK. 

通过 Microsoft Exchange 发送后的内容：

Dear Customer

Please find attached a comprehensive explanation of how to get our brochure of A=gola. This has been sent to you at the request of Alex.

The information has been taken from www.example.co.uk  "Company name" is one of the leadi=g tile and marble companies in the UK. 

正如你所看到的，由于某种原因，一些“n”字符被替换为等号“=”（例如：安哥拉>A=gola）

我的电子邮件配置：

$this->load->library('email');
$config['charset']      = 'utf-8';
$config['mailtype']     = 'html';


// SMTP
$config['protocol']     = 'smtp';
$config['smtp_host']    = 'exchange.example.com'; //ssl://
$config['smtp_user']    = 'email@example.com';
$config['smtp_pass']    = 'password';
$config['smtp_port']    = 25;

$this->email->set_newline( "\r\n" );

$this->email->initialize( $config );

$this->email->clear();

......

$this->email->from( $frome, $fromn );
$this->email->to( $email );

$this->email->subject( $subject );
$this->email->message( $send_message );


$this->email->send();

有谁知道为什么微软交易所会这样表现？还是我应该使用某种设置？