Address in mailbox given [username] does not comply with RFC 2822, 3.6.2.
I managed to solve it by editing my .env file, which I have located here:
/var/www/ninja/.env
For my SMTP settings, I had entered just my username for MAIL_USERNAME, which does suffice for my ISP but it seems that Invoice Ninja requires the first email address for the username.
The fully RFC 822 compliant regex is inefficient and obscure for validate email address because of its length. Fortunately, RFC 822 was superseded twice and the current specification for email addresses is RFC 5322. RFC 5322 leads to a regex that can be understood if studied for a few minutes and is efficient enough for actual use.
If you use HTML5, use this code:
<input type="email" name="email" required placeholder="Enter a valid email address">