If you don't require SMTP authentication, then many servers will reject your e-mail because you are an OPEN RELAY, more commonly known as SPAMMER HEAVEN.
Many modern e-mail servers now look for an "authenticated as" line in the message header.
They also look for DomainKey and DKIM entries, and compare them against the DNS records for the domain. These are modern antispam tools, and help eliminated the, more than, 95% of the e-mail traffic which is junk mail so it doesn't clog up in boxes and waste people's time
Setup your SmarterMail servers to require SMTP authentication for EVERY message zent through them, by EVERY DOMAIN, and EVERY USER ACCOUNT, or your e-mail will be rejected by all of tge large providers, spammers will discovwr your resources, and your IP ADDRESSES will be black listed in every spam database.
Bruce Barnes
ChicagoNetTech Inc
brucecnt@comcast.net
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Website: https://www.ChicagoNetTech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting