You can setup a plugin to make use of SMTP, then configure WordPress to connect to external SMTP server so messages are sent via this server.
Another option is to setup “relayhost” whereby you relay mail via your local Postfix installation to an external server.
It highly depends on how you are doing things, which solution would be most appropriate. Additional steps may be necessary.
As an email specialist, I can assist you in getting things setup to meet your needs if you would be interested.
*** this would be a billable task, but my rates are affordable, and you’re assured a solution before paying. ***
Peter Knowles | TPN Solutions
Need Professional, Affordable Tech Support? Request Assistance at: https://tpnassist.com