Depending on the email service provider, the default SMTP port number may differ. To be sure you're using the right port number, check with your carrier or use the Mail Settings Lookup to double-check your email settings. You can try to change the SMTP port numbers on your iPhone X after you have the relevant information. Here's how you do it:

1. Go to Accounts and Passwords under Settings.
2. After that, tap the email account you'd like to update.
3. Then, touch the SMTP and SMTP server you wish to change.
4. Use the port number provided by your carrier if you've chosen the option to Use SSL. Otherwise, turn off the Use SSL option and try another SMTP port, such as 80, 3535, or 25. Check whether these ports is compatible with your email account.