Sendmail auth issues

So been awhile since I have been on the forum but I need help and feedback outside of the HC support.
 
We have been running secondary Linux server as part of a cluster on the current HC  ver and are having issues in regards to email on this server. We currently have sendmail and have the following issues.
 
1) Sendmail will not accept sending outbound smtp with authentication with the users U/P. HC told us to use pop before smtp which back several years ago this was not an issue but as we get more user they want to configure their Iphone/Android, etc. and since none of the devices support pop before smtp they can't get it to work.  So when asking HC support recently  I got the normal did you install sendmail? WHAT? So I saw where this is going.
 
Pop before smtp has/is functioning with email clients but I have even seen the random issue where it does not work, just sends back error unable to reply sometimes and does not matter which version of Outlook you use.
 
2) IMAP to the mailbox. If we test Outlook against our Linux server and it destroys the mailbox. When you first open Outlook it will display the emails but when you try and open an email comes back saying that it disconnected and will not display anything anymore. Going back in via squirrelmail it tells me that the mailbox is corrupt. Opening the mailbox in an text editor it shows a couple of lines of html code and then is it cut off. Delete the mailbox file and then it is good through webmail and pop/smtp. So it looks like IMAP is out of the question.
 
3)At random a users will start getting the same emails over and over and over coming into the mailbox. The only way we have been able to stop and correct the issue is to completely delete the mailbox. After a couple of times of this happening we have lost the customer. HC says this is sendmail issue and that you have to address the issue. I have even contracted someone outside to look at the issue and they gave up since HC is using their own shadow and password files(non-standard config I was told) .
 
HC said to use Postfix "Because it is easier to configure" that did not answer the question on what to do so I am back at square one.
 
General questions to the Forum.
 
1) Do you use sendmail or Postfix?
2) Can you smtp with authentication off the users U/P?
3) Can you share how you did this?
4) Anyone been able to get a option to change the email password via the webmail?