The web project is coded by our own. No Wordpress or other possible points of security holes.
Our project does NOT send any email at all. I have even the sendmail binary disabled from PHP. All emails are sent by Sendgrid (and they are JUST emails of “registration” or “password recovery”). We send about 5 emails per day with this service. So all the emails coming out from my domain are these ones.
Queue is always in 0, there’s no signs of “massive sending” and filled queue as in spam scenarios.
The only thing I have is tons of: dovecot_login authenticator failed for [188.8.131.52]: 535 Incorrect authentication data (set_id=XXXXXXXXX) (in XXXXXX a lot of tries)
So, in this scenario, I’m getting crazy … and I would like to go further and DISABLE SMTP at all.
But I need to leave working the POP3 (because I want just to receive emails to an account), and the forwarding service (so emails incoming to one account, are forwarded outside, to a gmail account)
Freshdesk shouldnt be a issue. Did you got any additional informations about why you got blacklisted? Personaly I think disable smtp isnt the right way to go, you should find out why your IP gets blacklisted, it could point to a bigger issue.