Big mail issue two ip address

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http://serverfault.com – I have two ip addresses on my server, First IP is assigned to my clients, second is assigned to me. Issue started today. My clients can not send mail via php from their ip address (shared ip address which are all of them using). Server is refusing it with error ERROR: Password not accepted from server: 535 Incorrect authentication data But via their desktop mail clients it can send mail, when they use my ip address for hostname (second ip of server assigned to me) they can send mail. help. (HowTos)