Hello,
I am under PLESK 10.3.1 and since a few days I can not send e-mail from my server after checking the log (psa-horde.log) I always find the same error:
HORDE [error] [imp] sendmail returned error code 127 [pid 11779 on line 754 of "/usr/share/psa-horde/imp/lib/Compose.php"]
#...