You may need to Update SMTP Authentication for iPhone, please refer to the steps below:
1. Open Settings.
2. Scroll down and select Mail.
3. Next, click on Accounts.
4. Select your email account, then choose your email address.
5. Select the "Account Setings" option.
6. Select "Advanced" at the bottom of your screen.
7. If you have MD5 Challenge-Response selected, change the option to Password.
Conclusion:
CRAM-MD5 SMTP authentication is not supported by the server, so switching to password-based authentication is necessary; this change is required because CRAM-MD5's challenge-response mechanism is not available, while password-based methods—when used over TLS—provide a secure and widely supported alternative.
Thank you.
