SMTP error 555 typically occurs when the mail server is unable to register the email addresses used within the FROM or TO fields of the email
It may typically happen due to incorrectly spelled email addresses or if the email address contains an unrecognized character when copied from an external source.
To resolve this issue, ensure that all the email addresses within the email are valid and do not contain any unrecognized syntax. Also, check the recipient's email address for misspellings.
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