Paperless error message

I am getting the following error message from Paperless when I try to resend an invoice using that module:

(I've changed the names of the email addresses for privacy purposes)

Error on 'SendMail'.

ChilkatLog:

  SendEmail:

    DllDate: Jan 19 2012

    UnlockPrefix: SBestSoftMAILQ

    Username: PH3:sage

    Architecture: Little Endian; 32-bit

    Language: ActiveX

    recipients:

      TO: [email protected]

      totalCount: 1

    --recipients

    renderToMime:

      createEmailForSending:

        xSigningAlg: sha1

        Auto-generating Message-ID

      --createEmailForSending

      renderToMime: Elapsed time: 15 millisec

    --renderToMime

    SmtpConnect:

      SmtpHost: mail.smtp2go.com

      SmtpPort: 2525

      SmtpUsername:

      SmtpSsl: 0

      StartTLS: 0

      SmtpAuthMethod: NONE

      Need new SMTP connection

      No SMTP login provided.

      No SMTP password provided.

      Skipping SMTP authentication because no login/password provided.

      checkForExistingConnection: Elapsed time: 0 millisec

      SMTP_Connect:

        Connecting to SMTP server mail.smtp2go.com:2525

        smtp_host: mail.smtp2go.com

        smtp_port: 2525

        smtp_user: NULL

        auth-method: NONE

        ConnectTimeoutMs_1: 30000

        calling ConnectSocket2

        IPV6 enabled connect with NO heartbeat.

        connectingTo: mail.smtp2go.com

        dnsCacheLookup: mail.smtp2go.com

        dnsCacheHit: 45.79.170.99

        GetHostByNameHB_ipv4: Elapsed time: 0 millisec

        maxTimeMs_connect: 30000

        myIP_1: 192.168.1.12

        myPort_1: 62398

        connect successful (1)

        socketConnect: Elapsed time: 0 millisec

        InitialResponse: 220 mail.smtp2go.com ESMTP Exim 4.96-S2G Fri, 31 Mar 2023 14:53:18 +0000

        initialResponse: Elapsed time: 797 millisec

        sendingHello: EHLO PH3

        sendEhlo: Elapsed time: 0 millisec

        helloResponse: 250-mail.smtp2go.com Hello PH3 [24.187.132.236]

250-SIZE 52428800

250-8BITMIME

250-DSN

250-PIPELINING

250-PIPECONNECT

250-AUTH CRAM-MD5 PLAIN LOGIN

250-CHUNKING

250-STARTTLS

250-PRDR

250 HELP

        helloResponse: Elapsed time: 16 millisec

        smtpAuthMethod: NONE

        login_method: NONE or already authenticated

        CONNECTED to ESMTP server mail.smtp2go.com:2525

        authentication: Elapsed time: 0 millisec

        ConnectionType: Unencrypted TCP/IP

      --SMTP_Connect

      checkOrMakeSmtpConnection: Elapsed time: 813 millisec

    --SmtpConnect

    SmtpConn_sendMime_FROM: [email protected]

    recipients:

      recipient: [email protected]

    --recipients

    smtpRecipients: Elapsed time: 484 millisec

    email_size: 247630

    smtpSendData: Elapsed time: 141 millisec

    Sent DATA terminator

    550 verify the sender domain or email to be allowed to send.

    smtpConversation: Elapsed time: 672 millisec

    Failed.

  --SendEmail

--ChilkatLog

We are using the same SMTP that is normally used for sending invoices using 'Invoice Data Entry', so we don't understand why this is failing.

Additionally, I thought it was just my computer, but I just tested it on another and the same issue occurred.  

How do we go about correcting this? Any information is appreciated.