Microsoft Exchange iPhone 6S Problems

Hello All,

Thank you for your time, much obliged. I'm having difficulties with a particular scenario on my iPhone 6S:

I can receive e-mails on Microsoft Exchange e-mail and view calendar, contacts, notes. However, I can't send e-mails it's always the same error: "Cannot Send Mail an error occurred while delivering this message" that e-mail stays in the Outbox and nothing happens further

I've checked the settings several times back and forth, I'm afraid nothing helps. Also, it's important to mention that I have this exact Exchange Account on my iPad Mini and it works perfect.

Phone model: iPhone 6S Plus 128 GB; iOS 9.21 (13D15);

Please share your thoughts, any help will be much appreciated!