Problem replying from Apple iPhone to emails received - (sent to a cherished domain name)

[#1]

iPhone email issue:

I have a cherished domain name 'cdn.com'. The domain is hosted and run via Plusnet. There are 2 mail boxes I essentially use: 'mail' and 'mark'. The first is for general stuff and the second is for my personal email. Each have the the same (shared) password. 

When I get mails from suppliers etc ie non personal ones I can reply via Mark@cdn but not from mail@cdn. So my personal in box gets clogged with stuff meant for mail@cdn when I start a conversation.

Reply emails from mail@cdn.com sit in my outbox on my iPhone. I have no option but to send them from another email account eg Mark@cdn.com 

The Apple email (?)'client App' is really straight forward to use (I find Outlook a real pain and counterintuitive). 

Essentially, why can I use only 'Mark' and not the other 'mail' to reply to incoming mail? 

NB I can send out on mail@cdn.com just not reply. My understanding is that all the correct outward settings are correct - but I'm only a amateur ...  

Thanks ! 

[#2]

@Brock23 

My first move would be to see if this worked when logged in to Plusnet webmail https://webmail.plus.net/. This will confirm whether or not this is a Iphone client settings problem.

[#3]

Yes - thank you ! No problems using webmail.

Do you happen to know whether there are specific settings for iPhones - if so what are they? I have entered what I believe to be the correct settings but like to use the Apple App for convenience (and as its more intuitive). They certainly work for 'Mark@cdn.com' {outward going and to replies received} 

When I respond from mail@cdn.com (either as a direct reply - or even as a forward with the same content) I get the responses:

Cannot Get Mail The mail server ''imap.plus.net'' is not responding. Verify that you have entered the correct settings in the account info in Mail settings 

and/or (more frequently): 

Cannot send Mail check the settings for the outgoing servers in Settings>Mail>Accounts.

Is this a red herring for some other issue ? Is a 'kick start' required ? 

Thanks in advance 

[#4]

@Brock23 

Is @cdn.com hosted by Plusnet, or hosted elsewhere with a redirect to Plusnet?

If hosted elsewhere, do they redirect to TWO Plusnet mailboxes ... or both to one?

Have you configured TWO mail accounts on your iPhone?

• mark@
• mail@

If there are two mailboxes in PN's mail system, their userID should be accountname+mark and accountname+mail each having their own password.

On the iPhone if the email you have received was to mail@ and you want to reply from mark@ - having started the reply, click on the cc/bcc/from address field and chose a different from account / address, complete your message and click send.

The error messages also suggest that you have not got one of the mail accounts set up properly.