I am also having this issue; osTicket was installed in a cPanel (1.17-git).
Emails I send from my support email come through as a MIME Format; emails sent to my 'Alerts' email are also in MIME Format.
I have setup 'Remote Mailbox' and 'Outgoing (SMTP)' as well as set the authentication methods, but still have the MIME issue. Further, my emails are getting marked as spam by my recipient's mail server (my personal email @live.com).
Here is what finally worked for me:
REMOTE MAILBOX
Hostname: ssl://imap.stackmail.com (from my host)
Port Number: 993
Mailbox Folder: INBOX
Protocol: IMAP (from my host)
Auth: Basic Authentication (with my account login info)
OUTGOING (SMTP)
Status: Enable
Hostname: smtp.stackmail.com (from my host)
Port Number: 587
Auth: Basic Authentication (with my account login info)
NOTE: I had to make sure to select 'Basic Authentication' and NOT 'Same as Remote Mailbox' as this threw an error.
Then, head to "Diagnostic" (under the "Emails" button) and send a test email; this should work for you.