mautic.ERROR: [MAIL ERROR] Expected response code 354 but got code "503"

[wlsfh]

Don’t know if this has been tackled, but I am experiencing the exact same issue.
I opened a thread here

Fresh install // Not sending emails, NL form

Credentials are fine. I also received the test email using ‘send test email’ in configuration/email settings. It looks like it’s happening when sending emails to external addresses. By the looks of things, it seems this is a pending issue? https://forum.mautic.org/t/mautic-error-mail-error-expected-response-code-354-but-got-code-503/18705