SMTP error "Server does not support sending internationalized header data"

[ChristophWurst]

Steps to reproduce

0. Have a server without the SMTPUTF8 (RFC 6531)
1. Open the app
2. Send a message to chrístóph@domain.tld
3. Watch the outbox

Expected behavior

Message is sent

Actual behavior

\Horde_Smtp::send throws "Server does not support sending internationalized header data"

Mail app version

3.0

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Web server

None

Database

None

Additional info

No response