电子邮件组地址，例如undisclosed-recipients:;或a group:<[email protected]>,<[email protected]>;是根据RFC 5322的有效寻址形式，例如参见附录 A.1.3 中的说明性示例。在 Sendmail 中，这称为“列表语法”。最小的工作示例（对于 Bash）：
echo "Date: $(date --rfc-2822)
From: <[email protected]>
To: a group:<[email protected]>,<[email protected]>;
" | sendmail -t
但是，尝试使用空组提交消息会导致错误List:; syntax illegal for recipient addresses。最小的例子：
echo "Date: $(date --rfc-2822)
From: <[email protected]>
To: undisclosed-recipients:;
Bcc: [email protected]
" | sendmail -t
返回
undisclosed-recipients:;... List:; syntax illegal for recipient addresses
通过 提交邮件时，如何将 Sendmail 配置为支持空的 RFC 5322 组地址sendmail -t？