Is this the error you're getting from your postfix server? Can you test sending a message without it an use another phone or email address to see if the messages go through?
I think you may find an answer on this thread super helpful with your situation:
Sender Policy Framework 550 status message is part of spam validation screening. "550" is part of a long list of SMTP status codes to help diagnose connection issues. Mail relays connecting to @tmomail.net DNS and MX (mail) records must match or be authorized on behalf of another domain. The mismatch can occur in the registered domain or the shared ISP network connection.
This very simple FAQ provides a very high level understanding of SPF 550
This can occur when a company or university has an affiliate brand or satellite university, attempting to send email on behalf of the parent company. The authorized administrator for the parent company or university, needs to update DNS and MX records permitting all authorized affiliate mail relays to send on behalf of the parent mail server. Otherwise, the assumed SAFE policy is to "reject" the mail until the record is corrected. SPF is a global standard policy and can be strictly enforced or relaxed to accommodate spam control. Most mail administrators already maintain current MX and DNS records to accommodate for all global mail hosts they may need to communicate with, but sometimes there are exceptions or missed segments as companies grow or change hands.
With that said, SPF 550 response codes are intentionally sent back to the message originator to alert the sending party their DNS/MX host records are not up-to-date or mismatched. SPF prevents any rogue entity from spoofing the reply to: envelope on the received outgoing message sender.
T-Mobile cannot update DNS and MX records on behalf of sending mail servers, only the authorized administrator can perform this task.
To get this fixed, you'll need to contact Customer Care and a ticket will have to be filed for "blocked emails to @tmomail.net".