Setup postfix and dovecot for domain

I cannot get my mail working with virtualmin, dovecot and postfix

I have tried every setting. If I try to authenticate via my mailclient, I get an error instantly

If I try webmail (roundcube) I get: ‘SMTP authentication failed’.

I already checked:

• If dovecot was running under port 143
• if the mailserver I use (mail.mydomain.com) points to the correct IP
• if the user is listed in virtualmin under my domain
• I did re-check configurattion and everything is OK
• Sending mail from usermin is also not working, gives 'Failed to send mail : SMTP command . failed : 451 4.3.0 Error: queue file write error ’
• Sending mail to the mailadres is not working too (no error in mailclient, just not sent or received)
• I have set-up this DNS records: mail.mydomain.com. , pointing to MY vps-s IP (A record) and an Mx record vor mail.mydomain.com.
• In webmin I checked if both dovecot and postfix are up&running
• I tried numerous other settings but nothing seems to work

What now? Where do I start debugging?

PS: I don`t now if this matters, but the VPS mainserver (the domain I talk about here is a subserver), the mainserver is a subdomain like: srv5.myserversdomain.com. This hostname (srv5) points to the VPS. On the VPS I run virtualmin and withing virtualmin I have srv5.myserverdomain.com as mainserver and mydomain.com as subserver. The website is working fine for mydomain.com, just the mail not.

PS, I tailed the log too this gives 2 errors all the time: > queue file write error > hash: /etc/postfix/bcc is unavailable. open database /etc/postfix/bcc.db: no such file or dir > sender_bcc_maps_lookup porblem

Thanks a lot!