Invite a friend

The "Invite a friend" isn't operate. I get this message:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

****@gmail.com
SMTP error from remote mail server after MAIL FROM:<****@gmail.com

help please!

Invite a friend does work, this is nothing to do with Dolphin. It's a problem with your mail server.

There's plenty of solutions on Google.

but all kind of mail I can send (e.g through my pichosting)

Normally I would say your on the Spamhaus list (I think that's how it's spelled) but it seems that your getting nailed at the start section which is what's killing you.  So for this, I'd check with the host.  Has nothing to do with Dolphin, this is a domain/server related issue and needs resolved at that end.

the host provider is hostgator and I do not do anything anytime (the mail sending is good all my site on this host), so I don"t know what to do now (what I have to change)

You have been told what to do.

It does not matter that your other sites are working on that host. The problem is specific to the address or rather the domain the mail is being sent to.

Dolphin does not send the emails. Your host does. Dolphin just passes the email to your host to be sent.

Thus, you need to show a copy of the bounce back message to your host. They need to track down the problem.

Because everyone here is right. It is a mail server issue. And the mail server is handled by your host. So you have to talk to them.