Hi Channelzer0
It does work 4 me. Not sure if I know how I have made it running 
In docker settings please define variable MAIL_RELAY_HOST with the IP address of your Synology NAS.
I assume You have MailPlus server fully working installed on that storage device.
I hope, I have made myself clear 
Wish You luck !
P.S.
Pity is, I have not found solution for incorrect time zone (CET timezone in @ notifications - #12 by ilexpl01) in @ notifications.