[horde] sendmail returned error code 12

[Luis Felipe Marzagao/Andamentos]

Some users can send e-mail, others don't... They get this error:

HORDE [error] [imp] sendmail returned error code 12 [pid 10980 on line 
720 of "/home2/azevedom/public_html/horde/imp/lib/Compose.php"]


The configurations are exact the same for those who can and cannot send 
e-mail...

I can't find any info about this error code.

Any help?

Tks