Synology NAS Mail server relay error

Question

I am running Synology NAS and I want to use it as "man-in-the-middle" mail server.

Domain and main mail server is provided by ISP. I have there two mailboxes.

I was able to setup Mail Server to download emails via POP3 from ISP to NAS. Unfortunately I cannot figure out how to do the same with outgoing emails.

SMTP section:

Check Enable SMTP
Check Enable SMTP Authentication
Hostname (FQDN) mydomain.com
Port 25
Not checked “Enable SMTP-SSL”
Not checked “Enable SMTP-TLS”

SMTP Relay:

Check “Enable SMTP Relay”
server “smtp.provider.com” 
port 465
Check “Always use a secure connection (TLS)”
Check “Authentication required”
username and password same as for mailbox at ISP

With this setup I am able to send emails to myself and to my other mailbox, but any mail which goes outside is immediately returned with error

Server error: 554 5.7.1 : Relay access denied

when send via Outlook, when I send it from roundcube it says send and the mail goes to que. Afterwhile there is a info Connection timeout.

I have tryed all combinations at SMTP relay setting, unfortunately with no luck.

any tips what can I try please? Thank you very much