Let's say /var/log/maillog contains something like this.

Feb 18 00:20:42 ip-172-31-19-227 postfix/spawn[20917]: fatal: request to deliver as mail system owner

This may be an issue between Postfix and Sender Policy Framework (SPF). Notice in this example the maillog has "spawn". Here are more details on spawn - https://www.postfix.org/spawn.8.html

spawn is the command set in /etc/postfix/master.cf. Notice also that the maillog says "request to deliver as mail system owner". In this example, master.cf has "user=postfix", the mail system owner

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
policyd-spf  unix  -       n       n       -       0       spawn
   user=postfix argv=/usr/bin/python /usr/libexec/postfix/policyd-spf

This can be resolved by updating master.cf to have user=nobody.

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
policyd-spf  unix  -       n       n       -       0       spawn
   user=nobody argv=/usr/bin/python /usr/libexec/postfix/policyd-spf