SMTP Error (220): Authentication failed when sending from Roundcube

Hello all,
I've just finished installing ISPConfig 3.2.2 on Ubuntu 20.04.2 LTS using The Perfect Server tutorial and following it to the letter. In fact I ended up formatting the drive and doing it twice, just in case I missed something.
So far everything seems to be working perfectly, with one exception. And that is when I try and send an email with Roundcube, I get an "SMTP Error (220): Authentication failed" error.
I can receive mail, and I can send/receive mail using other mail clients, but I just can't seem to send with Roundcube, and it seems to be an issue with STARTTLS, based on on the mail logs.

Code:

Mar  3 12:45:04 myserver postfix/submission/smtpd[3595]: connect from localhost[127.0.0.1]
Mar  3 12:45:04 myserver postfix/submission/smtpd[3595]: lost connection after STARTTLS from localhost[127.0.0.1]
Mar  3 12:45:04 myserver postfix/submission/smtpd[3595]: disconnect from localhost[127.0.0.1] ehlo=1 starttls=1 commands=2

My Linux knowledge is ok, but I am by no means an expert.
I'm using PHP 7.4.3, and as I mentioned, everything else was setup exactly as per the instructions in The Perfect Server tutorial.
My apologies if I have missed some vital info in this post. Just let me know if there's anything I can add that will assist.
Thanks in advance,
Bruce

 

Is it? I see mention of an "SMTP Error (250)" error, but not "SMTP Error (220)". Are they the same issue?

 

Well I followed the steps for the 250 error, and I'm pleased to say that it worked. Perhaps worth adding to the "Read before posting" thread that it works for a 220 error as well.
I guess the "Perfect" server tutorial ain't so perfect after all.
Many thanks for the assistance.