Forwarded to devs Frequent email rejections with "SMTPUTF8 is required, but was not offered by host <hostname>"

[Bitpalast]

Username: Peter Debik

TITLE

Frequent email rejections with "SMTPUTF8 is required, but was not offered by host <hostname>"

PRODUCT, VERSION, OPERATING SYSTEM, ARCHITECTURE

Obsidian, latest MU
CentOS 7.9

PROBLEM DESCRIPTION

Since the latest Plesk Obisidian, an increasing number of emails that are mailed out through our servers are rejected by the recipient hosts with this error message:

SMTPUTF8 is required, but was not offered by host <hostname>

STEPS TO REPRODUCE

Send an email to a web.de or t-online.de account.

ACTUAL RESULT

Error message returned from the recipient's host.

EXPECTED RESULT

No error, delivery as usual.

ANY ADDITIONAL INFORMATION



YOUR EXPECTATIONS FROM PLESK SERVICE TEAM

Confirm bug

 

[Bitpalast]

Also happening with Posteo mail accounts.

See other forum posts on this like Issue - SMTPUTF8 is required, but was not offered by host

 

[IgorG]

Thank you.
Bug confirmed PPPM-12866

 

[Bitpalast]

The problem originates in the "Third-Party Component Updates" that were done with the update to Obsidian 18.0.34. This installed Postfix 3.5.9, and in Postfix 3.5.x the default setting for the parameter smtputf8_enable is "yes" while many other providers don't support the UTF8 setting. So we could actually call it "advanced" over other provider's settings, and it might have been premature by me to file this as a bug.

It is probably not really a "bug". It's more an issue with a newer capability brought in by Postfix 3.5.9 that many other mail servers do not support. So probably with the update Plesk should have set the parameter
smtputf8_enable=no
after updating Postfix. But as the updates are through already, it is too late for this and the only viable solution is the manual edit of the Postfix main.cf file.

 

[IgorG]

Fixed in Plesk 18.0.35:

Mail delivery to IDN emails no longer fails. (PPPM-12866)

 

[futureweb]

Fixed in Plesk 18.0.35:

@IgorG May I ask how the Problem was solved? Currently we added "smtputf8_enable=no" to /etc/postfix/main.cf on our Plesk Servers as Customers heavily complained (IDN Domains are more common here in Austria) ... should we remove this Setting after 18.0.35 upgrade?

 

[IgorG]

May I ask how the Problem was solved?

After upgrade to 18.0.35 there should be:

# postconf |grep utf8
smtputf8_autodetect_classes = sendmail, verify
smtputf8_enable = no
strict_smtputf8 = no

 

[mow]

After upgrade to 18.0.35 there should be:

# postconf |grep utf8
smtputf8_autodetect_classes = sendmail, verify
smtputf8_enable = no
strict_smtputf8 = no

Problems with this approach:

• other postfixes that want to send an SMTPUTF8-enabled mail won't be able to deliver to us because we don't announce SMTPUTF8 capability anymore
• Outlook 2019 requires SMTPUTF8 when it encounters an IDN domainname and doesn't handle it well when the mailserver says it can't use that

I think it was a very bad design decision to introduce SMTPUTF8 without a possibility for conversion/fallback, making delivery fail if a message somewhere started as UTF8.