javax.mail.SendFailedException: Invalid Addresses;

nested exception is:
javax.mail.SendFailedException: 550 not local host est.fib.upc.es, not a gateway

Hi! I've a problem with SendMail, it occurs that I am sending a mail into an specific address. In some cases the mail is sent correctly and in other cases it gives me the error showed above.

I've tried to discover why and it seems that if i send the mail to that specific address with an address on the from field it is sent ok, but if I send the same mail with another addresses in the from field it does not arribe.

It is quite strange, for example, we hace 4 specific email addresses called @1, @2, @3 and @4:

from: @1
to:@2
Sent OK

from: @3
to:@2
Send KO

from: @2
to:@2
Send KO

from: @2
to:@1
Sent OK

Does it make sense?

Thanks

Enric