Issue with Thunderbird and Autodiscover.xml

Hi,

first of thank you for the project and the fantastic work!

I've setup email-autoconf on a test environment to check it out. Thanks to the docs everything was straight forward. Checking with email_autoconf_test.py script the values are returned as expected.

After that i tried setting up the mail account in Outlook. It fetches the configuration correctly but there i hit my first issue. Outlook errors and tells me that it cannot connect with the IMAP Server as the IMAP Server does not support any authentication methods that Outlook supports. Adding the account manually works as expected. What raised my interest was that the autodiscover.xml has the following settings:

  <Server>imap.domain.tld</Server>
  <Port>143</Port>
  <SPA>off</SPA>
  <Encryption>Auto</Encryption>
  <SSL>on</SSL>
  <AuthRequired>on</AuthRequired>
  <DomainRequired>off</DomainRequired>
  <LoginName>demian@domain.tld</LoginName>

Should SSL really be set to on here if we are using STARTTLS? For reference this is the settings.py:

# SENTRY_CONFIG = {
#     'enabled': True,
#     'dsn': 'https://******@sentry.io/1234567',
#     'environment': 'prod'
# }

DB_TYPE = 'none'

IMAP_SERVER_HOSTNAME = 'imap.domain.tld'
POP3_SERVER_HOSTNAME = 'mail.domain.tld'
SMTP_SERVER_HOSTNAME = 'smtp.domain.tld'

IMAP_SERVER_SSL = ''
IMAP_SERVER_PORT = '143'
POP3_SERVER_SSL = ''
SMTP_SERVER_SSL = ''
SMTP_SERVER_PORT = '587'

EMAIL_PROVIDER = 'domain.tld'

The AutoConfiguration tester in Outlook works just fine and returns the expected information like ports and servername correctly.

The next issue i run into is that Thunderbird seems to ignore the autoconfig completely. KMail works just fine, but Thunderbird falls back to guessing method. I've studied the source code of Thunderbird to find some more details about the http / https situation but they use http as expected as long as mailnews.auto_config.fetchFromISP.sslOnly is false which is the default setting.

I'd like to use the project on a bigger mailserver but i can't seem to figure out what's going wrong. Any help or hints are appreciated!