gmx email error and gmx.com addy doesn't exist?

[CharleeBG]

I've been using Calibre for about a year with a very simple and basic understanding of how it works. I got a gmx email address and have been happily emailing docs to Amazon & my Kindle until a couple of weeks ago. Now none of the emails are going through, gmx says my email address doesn't exist, and I get this error when I try to create a new gmx account:
A technical error has occurred. Error Code: d3960a93-899e-4cbb-bd8f-9359cd4b45fb

Is it me or has gmx gone bananas?

This is an example of the error I've been getting from Calibre:

Spoiler:

calibre, version 4.18.0 (darwin, embedded-python: True)
Failed to email book: Failed: Email Haunted Destiny to charleegriffith_4fa69a@kindle.com

Starting job: Email Haunted Destiny to charleegriffith_4fa69a@kindle.com
connect: (u'mail.gmx.com', 587)
connect: (u'mail.gmx.com', 587)
reply: '220 gmx.com (mrgmx105) Nemesis ESMTP Service ready\r\n'
reply: retcode (220); Msg: gmx.com (mrgmx105) Nemesis ESMTP Service ready
connect: gmx.com (mrgmx105) Nemesis ESMTP Service ready
send: u'ehlo localhost.localdomain\r\n'
reply: '250-gmx.com Hello localhost.localdomain [173.218.133.179]\r\n'
reply: '250-8BITMIME\r\n'
reply: '250-AUTH LOGIN PLAIN\r\n'
reply: '250-SIZE 69920427\r\n'
reply: '250 STARTTLS\r\n'
reply: retcode (250); Msg: gmx.com Hello localhost.localdomain [173.218.133.179]
8BITMIME
AUTH LOGIN PLAIN
SIZE 69920427
STARTTLS
send: 'STARTTLS\r\n'
reply: '220 OK\r\n'
reply: retcode (220); Msg: OK
send: u'ehlo localhost.localdomain\r\n'
reply: '250-gmx.com Hello localhost.localdomain [173.218.133.179]\r\n'
reply: '250-8BITMIME\r\n'
reply: '250-AUTH LOGIN PLAIN\r\n'
reply: '250 SIZE 69920427\r\n'
reply: retcode (250); Msg: gmx.com Hello localhost.localdomain [173.218.133.179]
8BITMIME
AUTH LOGIN PLAIN
SIZE 69920427
send: AUTH <censored>
reply: '535 Authentication credentials invalid\r\n'
reply: retcode (535); Msg: Authentication credentials invalid

Sending failed...

Traceback (most recent call last):
File "site-packages/calibre/gui2/email.py", line 45, in run
File "site-packages/calibre/gui2/email.py", line 135, in sendmail
File "site-packages/calibre/utils/smtp.py", line 182, in sendmail
File "site-packages/calibre/utils/smtplib.py", line 645, in login
SMTPAuthenticationError: (535, 'Authentication credentials invalid')


Retrying in 301 seconds...

Waiting 301 seconds before sending, to avoid being marked as spam.
You can control this delay via Preferences->Tweaks
connect: (u'mail.gmx.com', 587)
connect: (u'mail.gmx.com', 587)
reply: '220 gmx.com (mrgmx104) Nemesis ESMTP Service ready\r\n'
reply: retcode (220); Msg: gmx.com (mrgmx104) Nemesis ESMTP Service ready
connect: gmx.com (mrgmx104) Nemesis ESMTP Service ready
send: u'ehlo localhost.localdomain\r\n'
reply: '250-gmx.com Hello localhost.localdomain [173.218.133.179]\r\n'
reply: '250-8BITMIME\r\n'
reply: '250-AUTH LOGIN PLAIN\r\n'
reply: '250-SIZE 69920427\r\n'
reply: '250 STARTTLS\r\n'
reply: retcode (250); Msg: gmx.com Hello localhost.localdomain [173.218.133.179]
8BITMIME
AUTH LOGIN PLAIN
SIZE 69920427
STARTTLS
send: 'STARTTLS\r\n'
reply: '220 OK\r\n'
reply: retcode (220); Msg: OK
send: u'ehlo localhost.localdomain\r\n'
reply: '250-gmx.com Hello localhost.localdomain [173.218.133.179]\r\n'
reply: '250-8BITMIME\r\n'
reply: '250-AUTH LOGIN PLAIN\r\n'
reply: '250 SIZE 69920427\r\n'
reply: retcode (250); Msg: gmx.com Hello localhost.localdomain [173.218.133.179]
8BITMIME
AUTH LOGIN PLAIN
SIZE 69920427
send: AUTH <censored>
reply: '535 Authentication credentials invalid\r\n'
reply: retcode (535); Msg: Authentication credentials invalid

Sending failed...

Traceback (most recent call last):
File "site-packages/calibre/gui2/email.py", line 45, in run
File "site-packages/calibre/gui2/email.py", line 135, in sendmail
File "site-packages/calibre/utils/smtp.py", line 182, in sendmail
File "site-packages/calibre/utils/smtplib.py", line 645, in login
SMTPAuthenticationError: (535, 'Authentication credentials invalid')

Job: "Email Haunted Destiny to charleegriffith_4fa69a@kindle.com" failed with error:
Traceback (most recent call last):
File "site-packages/calibre/gui2/threaded_jobs.py", line 83, in start_work
File "site-packages/calibre/gui2/email.py", line 102, in __call__
SMTPAuthenticationError: (535, 'Authentication credentials invalid')

Called with args: (u'/var/folders/77/1d4xxd4134l7m0xs62p805340000gn/C/calibre_4.18.0_tmp_UdFlbO/zzPxfBcaltmpfmt.mobi', u'Haunted Destiny - Heather Graham.mobi', u'charleegriffith_4fa69a@kindle.com', u'E-book: Haunted Destiny', u'Attached, you will find the e-book\n\nHaunted Destiny\n\tby Heather Graham\n\nin the MOBI format.') {u'log': <calibre.utils.logging.GUILog object at 0x12fa47150>, u'notifications': <Queue.Queue instance at 0x12f2f83b0>, u'abort': <threading._Event object at 0x12bb00f90>}

[kovidgoyal]

create a new gmx account or use hotmail instead.