Spam emails , SMTP got blacklisted why ?

[zillah]

Dear Experts

I have got a client who has a domain called : forum.com.nz (made it up) that is hosted by : APX Company (i.e. not his ISP provider).
Website designed by JDK Company not his hosting not his ISP provider.

Client internet provide is a nationalISP.

Client is using an outlook to retrieve mails for his domain (i.e. forum.com.nz)

POP3 : mail.forum.com.nz
SMTP : smtp.nationalips.com

Before the problem happened I realised there are a lot of spams flooding to his outlook.

All of a sudden the client called me that outgoing email stopped and when I tested his outlook there was an error message for the outgoing messages (not incoming), forgot what was that error.
Checked his public ip against mxtoolbox.com ,,,,,realized that it was blacklisted ,,,it took up to 73 hours to be relisted.

I want to understand the concept behind the problem; the reason for that before the problem of the spam hosting provider (APX company) told website designer company (JDK Company) that their security system caught suspicious codes.

My question if the spam emails are flooding via the hosting provider’s server (APX company) to the client email (my case is outlook),,,, why the outgoing SMTP (public ip address) for the ISP provider got blacklisted ? Is that because that ip address was used for incoming and outgoing emails ? If this is the case why outlook testing showing okay for incoming but not for outgoing ?

Regards

[Geek-9pm]

Often SMTP is blocked. by local ISP.  Some ISPs will not let you have a private SMTP. The problem is so big they blacklist all but a few.

The solution is to use another service for outgoing mail. Ask your ISP about this.

[Rob Pomeroy]

The problem is a lot more complex than it sounds.  Is the client running an on-premises mail server?  If so, and particularly if the client is behind a broadband connection, it should consider using a smarthost server (all mail is delivered to a single upstream mail server which then handles ultimate delivery).  As Geek says, if you're looking at low volumes, it's best to use the email service provider's outgoing SMTP.  Much better than attempting direct delivery.

If we're talking about higher volumes of mail, companies like MailGun and SendGrid offer suitable services.  Personally I am the architect of an enterprise mail delivery system that sends ~ 100k monthly emails via SendGrid.  It's working very well, with minimal "noise".  That option does require good knowledge of email delivery principles though.  You're effectively becoming you're own ISP for mail delivery, albeit outsourcing the actual delivery mechanism.

[suman]

 There should be NO traffic from his public IP to anything other than his isp smtp server his using for outgoing, even then it should most likely be using TLS and not the standard 25.
 
But if his public IP was sending spam directly, an infected box on his network - then sure his public IP could get blacklisted - and once blacklisted its quite possible that the smtp server he is using as smarthost would block him from sending email through it.
 
since his outlook is using a smarthost (isp or other smtp server to relay its email) there is NO reason why 25 should be allowed outbound from his network - other than maybe too his smarthost IP..  Then again this should most likely be on a different port normally default to 587 or 465 if smtp with ssl, etc.
 
Your going to need to get his public IP off a blacklist if he wants to relay email through any smtp server that uses blacklisting.
 
And as already posted - actual details would be most helpful in helping you!!  not made up domains, etc.