Error 421 on Centos Postfix maillog

Hello, lately my hosting server's maillog is recording at least 4-5 of these errors per day:

/var/log/maillog:Feb  7 03:38:03 hostxxxxxxxxxx postfix/smtp[18250]: AC07EFD1F2: host mx.pcpuntodoc.it[62.149.128.157] refused to talk to me: 421 mxcmd05.ad.aruba.it bizsmtp FEgo1s00s2pLL4n01 Too many connections, try later.

I am on Centos 6.6, Apache and Postfix.

The server is only used to send mails from the various websites hosted there, such as order mails for e-commerce and so on.

I am unclear as to whether this error is related to my server or to the server receiving the message (more likely).
It mostly happens with two particular servers on the receiving end, the one listed above and another one, both managed by the Aruba italian service provider.

Is there anything I can do on my end, or should I simply overlook this?

Also, does anybody remember which are the error codes (equivalent to 421 in this instance) you receive when your server is blacklisted?

Thanks.