Issues with custom SMTP - 550 Mismatching sender domains

[Stou]

So I have been trying to setup a custom SMTP service on my Prestashop, but everytime I hit the test email button it throws the following error:

Expected response code 250 but got code "550", with message "550 Mismatching sender domains

So I have been trawling the forums for solutions but without any luck. Most posts seem to focus on Gmail.

Other posts suggests that a solution is to change the emails under Contact to the email you wish to send from, but no luck here for me either.

I am using Prestashop 1.7.8.7

I have tried resetting and setting things up multiple times in case of typo's and such. But I am running out of hints. Thoughts anyone?