feedmail-queue-alternative-mail-header-separator

feedmail-queue-alternative-mail-header-separator is a variable defined in `feedmail.el'.
Its value is

nil

Documentation:
Alternative header demarcation for queued messages.
If you sometimes get alternative values for `mail-header-separator' in
queued messages, set the value of this variable to whatever it is.
For example, `rmail-resend' uses a `mail-header-separator' value of empty
string ("") when you send/queue a message.

When trying to send a queued message, if the value of this variable is
non-nil, feedmail will first try to send the message using the value
of `mail-header-separator'. If it can't find that, it will temporarily
set `mail-header-separator' to the value of
`feedmail-queue-alternative-mail-header-separator' and try again.

You can customize this variable.