SUMMARY
Login to GMX SMTP does not work, Trojita does not recognize the 334 message after its AUTH PLAIN

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***


STEPS TO REPRODUCE
1. Add GMX as SMTP configuration (mail.gmx.net, STARTTLS or TLS)
2. Activate "Authentication" and add pretty much any email and password, hit "Save"
3. Compose a new email
4. Hit "Send"

OBSERVED RESULT
The progress bar stops at 50% for some time. The app waits for quite some time and finally declares a timeout.

Except from console log:
---
SMTP >>> "EHLO localhost\r\n"
SMTP <<< 220 gmx.net (mrgmx104) Nemesis ESMTP Service ready

SMTP <<< 250-gmx.net Hello localhost [84.xxx.xxx.xxx]

SMTP <<< 250-8BITMIME

SMTP <<< 250-AUTH LOGIN PLAIN

SMTP <<< 250 SIZE 69920427

SMTP >>> AUTH PLAIN
SMTP <<< 334

None of two regular expressions matched the input "334\r\n"
SMTP <<< 421 gmx.net Service closing transmission channel - command timeout
---

EXPECTED RESULT
Login is successful

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Lubuntu 22.04.2
(available in About System)
KDE Plasma Version: 5.24.7-0ubuntu0.1 / 5.92.0-0ubuntu1 (?)
KDE Frameworks Version:  5.24.7-0ubuntu0.1 / 5.92.0-0ubuntu1 (?)
Qt Version: ?

ADDITIONAL INFORMATION