Block requests from web browser and only allow from clients application in ASM

Hi iRule,

you are not giving away a lot of information, like clients coming from internal / external IP adress ranges, or type of mail server (Exchange, Zimbra,...). Therefore my recommendations cannot be very specific to your issue.

However, the solution for your issue doesn't require ASM. You can use iRules or Local Traffic Policy to block access to a specific URLs (HTTP::path) or to block access by source IP (IP::client_addr) or by User-Agent String ([HTTP::header "User-Agent"]).
Of coure you can also use ASM and either set a disallowed URL or an IP Address Exception (Always Block this IP).

A word of caution: Personally, I consider blocking by User-Agent string as rubbish. UA strings can easily be spoofed.

KR
Daniel