Do you have the username and password configured above that? If not, then you need to click the "Uses Separate Authentication". next to Authentication Required.

Other than that 535 usually means invalid credentials either by incorrect username and/or password. You can get with ionos and ask them what the format is for sending email via SMTP. I have 2 providers. 1 requires just a username and the other requires username@domain.com.

You would also likely want to talk to GoDaddy support about if they are blocking mail ports from their webservers.

I am writing the solutions here because I also encounter the problem and cannot find any solution online.
By tracing the source code, I realize that the OSticket uses Pear Mail library to send email.
There is a conditional testing in pear mail socket library causing the problem.

When a user type in
eg. smtp.ionos.de, it translate into IP address eg. 123.123.123.123
due to the defects of the checking, it intends to do the reverse but yet there is no update or fix for that library.

So, to overcome this, for SSL port 465, one may type in ssl://123.123.123.123 with protocol
Because it takes any '/' and will use the hostname without trying to resolve it into IP.
It works because at the last moment before sending out the command, it will add the protocol anyway for connection purpose.

Hope this could save someone else's time from trying.

In addition to previous reply, the Net Socket is now at PHP Version 5, the bug should be fixed already. OSTicket is still using PHP Version 4. I suggest OSTicket team to update its library for connection.

sorcererapp

Answered this on GitHub but answering on here as well. We are moving to the laminas-mail package and ditching the pear library altogether with the upcoming v1.17 release. This is in response to the Modern Auth changes most major mail providers are making in the coming months.

Cheers.