错误详情：
# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
*** ERROR: FEATURE() should be before MAILER()
*** MAILER(`local') must appear after FEATURE(`always_add_domain')*** ERROR: FEATURE() should be before MAILER()
*** MAILER(`local') must appear after FEATURE(`allmasquerade')*** ERROR: FEATURE() should be before MAILER()
解决办法：
按照提示，编辑 /etc/mail/sendmail.mc 文件，将 FEATURE() 放到 MAILER() 之前：
更改
dnl #
dnl # Default Mailer setup
MAILER_DEFINITIONS
MAILER(`local')dnl
MAILER(`smtp')dnl
dnl # Masquerading options
FEATURE(`always_add_domain')dnl
MASQUERADE_AS(`tumblr.3sd.me')dnl
FEATURE(`allmasquerade')dnl
FEATURE(`masquerade_envelope')dnl
为
dnl #
dnl # Masquerading options
FEATURE(`always_add_domain')dnl
MASQUERADE_AS(`tumblr.3sd.me')dnl
FEATURE(`allmasquerade')dnl
FEATURE(`masquerade_envelope')dnl
dnl # Default Mailer setup
MAILER_DEFINITIONS
MAILER(`local')dnl
MAILER(`smtp')dnl
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