邮件反弹(退信)：使用独立域发信时需要配置MX

邮件群发讲究发件人信誉，因而客户比较热衷使用自己的独立的发信域来发信。那么在域名配置时候需要注意配置投递服务商的MX。因为有些收件服务商会查看发件人的邮箱的MX是否有效存在，以访垃圾邮件。

退信如下：

"message": "5.1.8 <bounce+72342a.4bd44a-lindalewis213=bellsouth.net@mta.xxx.com>... Domain of sender address bounce+72342a.4bd44a-lindalewis213=bellsouth.net@mta.xxx.com does not exist"

投递服务商回答：
The error (which is called Sender Address Verification) that you are seeing is due to the recipient server performing an MX lookup on the envelope.mail-from address and failing to find the records for the subdomain mta.xxx.com:

https://mxtoolbox.com/SuperTool.aspx?action=mx:mta.xxx.com

Some email servers check the subdomain in this envelope.mail-from field for MX records, and if the records do not exist, the email servers reject the message. Therefore, by adding Mailgun's MX records to your subdomain, the MX lookups of those email servers should succeed, and the servers will accept your messages. The two MX records for Mailgun are:

Value Priority
mxa.mailgun.org 10
mxb.mailgun.org 10

Reference: https://documentation.mailgun.com/quickstart-receiving.html#add-receiving-mx-records
Once DNS propagates for this domain (up to 48 hours), the error should disappear. If not, please also try adjusting the From: of the message to be an address that matches this domain as well.