Some of the messages sent from the domain promarkets.com.ar are being classified directly as SPAM, or the recipient is being warned as possible SPAM. 

How can I check if there's any configuration missing. Or what can be the problem.

I have tested the domain with http://www.dnsstuff.com

and within the result I get these warnings related to emails:

1. Connect to mail server

Connections to the following IP addresses on port 25 failed. It is recommended that you ensure all mail servers are listening on port 25

204.141.33.55 | connected

204.141.32.121 | connected

8.39.55.177 | failed message send with: failed cx open with: failed socket connect with: Operation now in progress

2. SMTP greeting

Malformed greeting or no A records found matching banner text for following servers, and banner is not an address literal. RFC5321 requires one or the other (should not be a CNAME). If this is not set correctly, some mail platforms will reject or delay mail from you, and can cause hard to diagnose issues with deliverability. Mailserver details:

204.141.33.55 | WARNING: The hostname in the SMTP greeting does not match the reverse DNS (PTR) record for your mail server. This probably won't cause any harm, but may be a technical violation of RFC5321

204.141.32.121 | 250-mx.zohomail.com Hello dnsstuff.com (adf-b.dnsstuff.com (74.115.12.14))250-STARTTLS250 SIZE 53477376

3. SPF value covers incoming mailservers

The SPF value does not allow mail delivery from all mailservers in the domain. This may cause problems if you use the same servers for incoming and outgoing mail. The failing mailservers are:

transitioning domain of promarkets.com.ar does not designate 8.39.55.177 as permitted sender | softfail | 8.39.55.177

Could you assist me on how to solve them?

Looking forward to your soon reply.