Symptoms
You may come across bounce-back emails with this error when sending emails to certain remote email servers.
Description
The issue is that the remote email server does not accept standard SMTP commands like MAIL FROM. We have seen this issue happen with Coremail mail server software.
Workaround
Reach out to the administrator of the domain you can't send email and ask for proper syntax. Utilise the WHOIS database to find the administrator email for a domain.