SASL Authentication For Mail Server (Sendmail/Postfix)

I recently moved to virtual dedicated hosting for my website, mainly because I wanted a better mail server to work with to manage a mailing list.

I have a few PHP software scripts for different things: managing affiliates and email marketing.

I want to send mail using both of those via SMTP on my new server. They both have a place to type in the host and username/password.

The problem I am noticing is SASL Authentication. They attempt to connect to the host without any trouble. However, the affiliate software fails SASL Authentication every time I try to send an email or test the email system.

Email marketing software connects when I manually send a campaign. However, users are supposed to immediately receive a confirmation mail upon opting in. In this case, it shows up as an attempt in the mail log, but it fails SASL Authentication. I don't know if it's a problem with the software or not, since it attempts to connect but fails, yet connects successfully in other cases.

My question is this: Is there another way around this? I've contacted my host to double check on the appropriate username/password to pass authentication. I will also be contacting the software developers after I receive an answer.

But if nothing comes of it, is there any way around it? Is there a place where I can have it automatically authenticate if the email is coming from a certain user, or if the attempt is being made from the same server (everything sits on the same server, I use localhost in the software to connect to the mail server). Since I am the only person who has access to the server and uses it, it seems safe to automatically authenticate if the request is being asked by the server itself...

I'm just looking for other alternatives if the software does indeed have a problem and I cannot figure it out.

Keep in mind I am very new to all this server and root access. Just a few days ago I was on a shared hosting plan, so I didn't have any access to anything. With that in mind, I know very little about mail servers, although I've been learning more over the past few days. I only stumbled upon the mail logs on my server recently by browsing files in the control panel (I'm not familiar with SSH, although I do have access to it)