My IP is blocked as SPAM by Hotmail and Gmail

I have a service in Postfix but now the messages are not being delivered checked the mail queued and find that Gmail and Hotmail is rejecting my deliveries and I have marked as SPAM, do not use my drop to send spam, just personal mailings.

That I can do to fix it?

1EC48449F0 2454124 Wed Jun 17 22:08:07 info@mail.dnc (host[] said: 550 SC-002 (SNT004-MC1F9) Unfortunately, messages from XXX.XXX.XXX.XXX weren’t sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to (in reply to MAIL FROM command))

115B544456 1620 Wed Jun 17 10:32:59 info@otherdomain.dnc (host[] said: 550-5.7.1 [XXX.XXX.XXX.XXX] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit 550 5.7.1 for more information. w53si5629300qge.37 - gsmtp (in reply to end of DATA command))

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I have the same issue with my mail address.

We are a small rent a car company and when I tray to replay to my clients I get the error SMTP error from remote mail server after pipelined MAIL SIZE=6335: 550 5.7.1 Unfortunately, messages from [] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3140).

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Out of interest, was your email fully configured with SPF record and DKIM? Also, again just out of interest, were you using either Gmail or Hotmail as a relay and are you using either of their paid services, such as Gmail’s Apps for Business, or M$'s Office 365 for example, or are you using their free services? Also, again just out of interest, what port was you sending from? I know it’s a lot of questions… I’m just curious. Cheers and good luck getting things sorted.