I am trying to send mails with mailgun. My DNS config (SPF,DKIM) seems to be ok and are being validated in mailgun service. I can send mail to several users with gmail, live and most others mail providers. However, I have a problem when I sent an email for email accounts of my university.

The message is rejected with the following alert:

> 
> **"554 5.7.1 : Client host rejected: MX-CIDR"**
> 
> 
> 

My current DNS settings are:

```
TXT @ "v=spf1 include:mailgun.org ~all"
MX 10 mxa.mailgun.org.
MX 10 mxb.mailgun.org.

```

DKIM was validated as well. I checked my domain at mxtoolbox and the dns config pass in all tests. I did not find errors related with that alert in others questions. May someone help me to fix it?

**Update 1:**
Just some more informations: 

1) I dont send, and I have absolutely no intention to send spam. I created an educational website, used by students and instructors, and they send messages sometimes between each others. I also send mail to confirm registers, recovery password, as a lot of others websites do. I only send messages to people who was agreed with my terms of service, that includes the information about my mail policy. It is a small service, I never sent more than 2,000 messages in a month (I have 800 registered users so far)

2) I do not believe I was blacklisted, mxtools verify several blacklists databases and my IP have passed in all verifications. Also, the server is not rejecting all messages from my IP, I can send messages with my personal email with the same domain, but I use different services to handle my personal inbox with my domain and the emails send by my website. So, I guess it may be a DNS record mistake.

3) I only use mailgun (or others transactional email services like mandrill or sendgrid) because it is highly recommended (and easy). I use a small VPS and it is hard to configure my own email service (I am a programmer, I am not an expert in that kind of configuration). If exists negative factors about the use of these systems, I really like to know and learn more.