Email trouble after server migration

I’ve migrated our
Osticket v1.9.12 from a dedicated Ubuntu 14.04 server to FreeBSD 11.1
and I am not able to send or receive emails.

server info: https://imgur.com/a/JP9YD

Sending:

When sending a test
under Admin panel > emails > diagnostics it reports as “sent
successfully” and indeed it does, but according to the mail server
logs it osticket is sending it it from www@helpdesk.domain.com
which causes the below error

Nov 4 12:23:34 mx01
postfix/smtpd[30057]: NOQUEUE: reject: RCPT from
c-xx-xxx-xxx-1xxhsd1.xx.myispnet[xx.xxx.xxx.xxx]: 550 5.1.0
<www@helpdesk.domain.com>: Sender address rejected: User
unknown in virtual mailbox table; from=<www@helpdesk.domain.com>
to=<test@ypcr.com> proto=ESMTP helo=<helpdesk.domain.com>

How can I get
osticket to send from the address SMTP is configured as?

smtp settings: https://imgur.com/fqYRLdP

Receiving email:

I can’t seem to
pin point the cause, this is showing in the osticket logs.

error: https://imgur.com/a/CI9ju

Any help would be great.

note: domain.com = my actual domain.

Thanks,

Tim