unable to send emails from lineone email account on iphone 6s

What happens when you try to send mail? Do you see an error? If so, what is the exact text of the error? Sending mail is handled by the SMTP server. You may need to verify that all of the settings for the SMTP server are correct, according to your provider, including username, password, and any SSL or port settings.

When you set up the device, there are more than one place to enter your username and password. In the Mail app, some have the word Optional by them when you get to the screen, but many email providers require them.

Go into Settings>Mail, Contacts, Calendars>select the email account in question. From there, scroll down and find the Outgoing Mail Server section, and the SMTP. Tap on that, and it should bring you to the SMTP section. You should see a Primary Server. It should show On, and then tap on it. On that page, you should see the settings, including the name of the SMTP server, and a place for the Username and Password. Just to make sure, go ahead and put them back in there again. You may also see the section about Use SSL. Check with your provider to see if that should be on, and then what authentication you should be using if necessary and Server Port.

Then I suggest that you remove the account from the device, perform a reset, which will not affect your data or content, by holding the sleep/wake and home buttons together until you see the Apple logo and then release. After the device reboots, try adding your account again, making sure you enter your username and password in all of the areas.

Okay, if you just re-created the mail account, if you go to Settings again and look at SMTP, do you have more than one there? Does this error occur on wi-fi and cellular, or only cellular, or only wi-fi. I would also double-check with the email provider to ensure the SMTP settings are correct.

So this only occurs on wi-fi? Is this email provider also your ISP? Do you have any other devices using wi-fi that you can test sending mail? Is this account a POP or IMAP account? What port are they using for the SMTP server?

You didn't say if you have more than one server listed now. Next, if you look at the settings on the iPad, do they match that of the phone?

So you didn't have more than one SMTP server from that same provider? Do you have more than one email account on the phone, that requires you to have multiple SMTP servers? I cannot understand why the settings on the iPad are different, but you say trying them will not work on the phone. What is different about the iPad settings?

Okay, this is what I was looking at originally. I have found when users remove and reinstall email accounts, at times the SMTP server does not get removed and can remain in the list. From there, the mail account can try choosing the wrong server. Now I'm even more confused why you would have a different server on the iPhone from the iPad for the same email account. It makes no sense. From the server address you provide, it appears you are in the UK. I'm located in the US, and are not familiar with this provider. You may need to contact them directly and work with them while you have the phone in front of you to see if you can get this sorted, or possibly someone else with this provider can assist. Seems we have tried all I'm aware of for checking and verifying your SMTP account, so something else is blocking your ability to send mail. Sorry I cannot think of anything else.