The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains addresses of recipients of the message whose addresses are not to be revealed to other recipients of the message. There are three ways in which the "Bcc:" field is used. In the first case, when a message containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is removed even though all of the recipients (including those specified in the "Bcc:" field) are sent a copy of the message. In the second case, recipients specified in the "To:" and "Cc:" lines each are sent a copy of the message with the "Bcc:" line removed as above, but the recipients on the "Bcc:" line get a separate copy of the message containing a "Bcc:" line. (When there are multiple recipient addresses in the "Bcc:" field, some implementations actually send a separate copy of the message to each recipient with a "Bcc:" containing only the address of that particular recipient.) Finally, since a "Bcc:" field may contain no addresses, a "Bcc:" field can be sent without any addresses indicating to the recipients that blind copies were sent to someone. Which method to use with "Bcc:" fields is implementation dependent, but refer to the "Security Considerations" section of this document for a discussion of each.
This function will go to the next line of non-quoted text which comes after a line of quoted text in the internal pager.
以及用于切换引文是否显示的：
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<toggle-quoted> (default: T)
The pager uses the $quote_regexp variable to detect quoted text when displaying the body of the message. This function toggles the display of the quoted material in the message. It is particularly useful when being interested in just the response and there is a large amount of quoted text in the way.
Reply-To
这个邮件首部为用户回复邮件时提供建议，设定新邮件的To:首部。
Thread
References
In-Reply-To
format=flowed
Mail-Followup-To
回复邮件时如果选择了follow up(或者说reply to
all)的方式，那么收件人会被填为Mail-Followup-To设定的地址。
避免发件人如果订阅了该邮件列表的话收到两封相同的邮件。