|I am receiving several timeout error messages from the F-PROT milter.|
If the milter plug-in seems unresponsive to Sendmail, it will cancel the pending job. Sendmail will then return "451 Please try again later" to the peer mailserver to indicate it has a temporary problem in processing e-mails.
Sendmail's default timeouts used are insufficient for f-prot-milter. Make sure you have this as your configuration line in sendmail.mc (to activate the change you must compile a sendmail.cf file and restart sendmail):
INPUT_MAIL_FILTER(`f-prot-milter', `S=inet:email@example.com, F=T,T=E:8m;C:5m;R:4m;S:2m')