Bulk Email Server

Bulk e-mails on their own will not cause blacklisting - sending unsolicited mail will however and you will not prevent blacklisting if that is what your client is doing. Make sure they are using a double opt-in list and there shouldn't be any problems.

 

Any mail server can get blacklisted given sufficient reports. Basically, if your clients are sending legitimate requested messages then you shouldn't have any problems, however if it's unsolicited then personally I'd drop them as it's not worth the hassle of trying to avoid blacklisting and if you are, you're basically encouraging spam mail.

 

Most decent mailing list packages support double opt-in as it's one of the easiest methods to ensure that a recipient genuinely wants to receive mailings - phplist is an example of this. What sort of mailings is your customer trying to perform though? Are they genuine mailings or marketing?