email lacks sender address

[Haines Brown <haines@histomat.net>]

I use mutt to send messages through exim4. Some messages lack a
sender. Here is a log emtry showing that from: address is blank:

Apr 24 11:11:29 tev-mail-relay1 postfix/smtpd[510179]: connect from
  unknown[32.210.108.191]
Apr 24 11:11:30 tev-mail-relay1 postfix/smtpd[510179]: Anonymous TLS connection
  established from unknown[32.210.108.191]: TLSv1.3 with cipher
  TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256)
  server-signature RSA-PSS (2048 bits) server-digest SHA256
Apr 24 11:11:30 tev-mail-relay1 postfix/smtpd[510179]: NOQUEUE: milter-reject:
  MAIL from unknown[32.210.108.191]: 554 5.7.1 Empty Sender Address
  is prohibited through this server; from=<>
  proto=ESMTP helo=<lenin.histomat.net>
  sasl_username=<brownh@historicalmaterialism.info>
Apr 24 11:11:30 tev-mail-relay1 postfix/smtpd[510179]: disconnect from
  unknown[32.210.108.191] ehlo=2 starttls=1 auth=1 mail=0/1 rcpt=0/1
  bdat=0/1 quit=1 commands=5/8

521 5.5.1 Protocol error (154.24 ms)
Unverified address

I reconfigured exim4 and it has no problem.