There are few points to note here:
First of all, MX records only affect receiving email, not sending email. The fact that you can receive email means your MX record is correct. Whatever is preventing you from sending email is clearly unrelated to your MX configuration.
There seems to be some sort of problem with MXtoolbox and retrieving MX records from our nameservers. I don’t know why that happens, because other DNS lookup tools work fine:
As for sending email, can you please clarify how you sent the email and where you were unable to receive it?
A good place to start debugging email sending is http://mail-tester.com . Simply create an address there, send an email to it and check the results (or not, if the email is never delivered). If you need help interpreting the results, just share the URL here.