Email Solution on Windows 2012 IIS Server

We have a PHP app running along side our original classic .asp application on a WIndows 2012 R2 server running IIS.  In our original app we used MailEnable (free version) to allow us to send emails out.  However this solution is no longer working for us.

Here is a brief summary of what happens... the users have their own email addresses - which they prefer to send FROM - and they want to send updates and invoices to their clients.  Ten years ago sending "From" their own email addresses was not much of a problem.  Now with the spam filters and other issues, many of these emails fail to send.

Also many of our users are still maintaining their original public email addresses - lots of Yahoo, AOL, etc. email addresses.  This too, is becoming an issue.

As we have tried sending FROM their email address TO their clients, deliverability has dropped significantly.  The sending IP doesn't match the FROM domain IP and won't deliver - even though it's not spam or any deceitful type of email.  These are legitimate emails needing delivered from our users to their clients.

So what I am asking for is suggestions as to HOW to get emails delivered to the clients, and still keep our users happy.  We did setup a specific email address to be sent FROM so the sending email matches the sending IP, as well as also setting the REPLY TO address back to the user's email itself.  However, the users panic when their clients ask if they have a new email address (our standard email used by all sends).  They don't want their clients sending to that email address and then not receive their emails.  We too, would have no idea which client belonged to which user.

We have thought about using a third-party SMTP service for deliverability, but then again they do not allow for the use of public email addresses either.

Suggestions anyone?  Of course keeping it as low-cost as possible.  Currently we are stuck between a rock and a hard place converting our classic app to the new PHP format.  So money is tight, but we definitely need a solution ASAP.

TIA!
OverthereAsked:
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Adam BrownConnect With a Mentor Sr Solutions ArchitectCommented:
I'm afraid you're going to be stuck with a complete redesign or change to your application's methodology. The only way you're going to be able to reliably send messages using someone's address is to actually send the messages through that individual's actual mail servers. For instance, if you have a client that sends mail from a yahoo email address, your application will need to connect to Yahoo's email servers using your client's actual credentials to send the message. What this means is your application will have to be configured with mail credentials for each client's email service and when the client wants to issue an email, the application must connect and send the message using those credentials and email servers. Any other method you attempt will fail unless all of your clients' clients set things up to white list your sending servers. The reasoning here is that you're effectively making use of an extremely common spam technique, where you're spoofing your customer's email addresses when you send messages to their clients. As you mentioned, this wasn't such a big deal 10 years ago, but these days, no spam filter is going to deliver messages using those techniques reliably.

The big issue you're dealing with is that your clients regularly use Public email addresses. If they all had their own mail servers and mail domains, you could instruct them to add your IPs to their SPF records for email, and that would effectively improve the likelihood of your messages getting through. Unfortunately, Yahoo, gmail, and the like don't allow that type of thing. So you're going to have to redesign your application to request mail server IPs, ports, and credentials and then use those when sending email for things to work properly.
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OverthereAuthor Commented:
That makes complete sense and followed along with my thought, however I was just curious if there were any other alternatives available.  I do have a domain that I could assign each of my users an email address which "forwards" to their public email.  Are there still outstanding issues when the email is forwarded to a public address?  Again - need to know if this might be an alternative answer.  This would give the users an email belonging to the sending IP and be more personalized rather than sending FROM a single email address "On behalf of..." the user.
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OverthereAuthor Commented:
Thanks for your time and verifying my initial thoughts.
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Adam BrownSr Solutions ArchitectCommented:
Forwards can get caught up in spam filtering as well, but using a secondary domain would be a little more reliable I think
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