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sstricker
Level 1
October 20, 2026
Question

IP Address Delivery Issues During Email Send

Hi team,

While reviewing our recent email send using @email.triumph.io email domain (the newest one we setup for the second instance of marketo), we noticed a few of bounce messages and wanted to flag them for review.

Bounce Message #1: 5.7.134 RESOLVER.RST.SenderNotAuthenticatedForMailbox; authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this mailbox.

Bounce Message #2: 550 spamcop.mimecast.org Blocked - see https://www.spamcop.net/bl.shtml?199.15.214.50.
https://community.mimecast.com/docs/DOC-1369#550

Bounce Message #3: 5.7.1 XGEMAIL_0008 Command rejected

Bounce Message #4: 550 "JunkMail rejected - sjsmtp.mktomail.com [199.15.214.50]:53124 is in an RBL: "

It looks like:

  • Some recipients may be rejecting messages due to sender authentication requirements.

  • The sending IP (199.15.214.50) appears to be listed on the SpamCop blocklist, which is causing rejections from Mimecast-protected domains.

  • Not sure what the other two mean.

Could you please investigate and let us know:

  1. Whether there are any authentication issues with the sending domain (SPF, DKIM, DMARC, etc.).
  2. Whether the IP listing on SpamCop is known and if any remediation is needed.

  3. If there are any recommended actions on our side before we continue ramping up send volume.

 

Email campaign volume was 2.1k recipients, ~60 got hard bounce message. 

Thank you, 

1 reply

sandy_logitech
User Group Leader
sandy_logitechUser Group Leader
User Group Leader
October 20, 2026
  • The “authentication” referred to in Bounce Message #1 isn’t message or connection authentication in the sense of DKIM. It means the recipient’s mailbox requires you to be logged in to their internal network. You cannot solve this in Marketo; they likely put an internal-only mailbox on a form by mistake.
  • Unless you have the branded envelope sender add-on and are using email.triumph.io in the SMTP MAIL FROM (this is not the same as the From: domain, which I assume is what you mean by “using @email.triumph.io”) SPF makes no difference at all. This is a very common mistake.
  • I can tell you more about your envelope sender, DKIM signature, etc. if you send to the test address test-sstricker@load.flowboo.st.
  • Bounce Message #3 is from Sophos’s instance blocklist (i.e. administered by the Sophos customer). It suggests your domain was deliberately blocked for abuse.
  • SpamCop is one example of an RBL (we usually say DNSBL these days). Bounce Message #4 means the IP was on a blocklist at that point in time.
  • The SpamCop listing is transitory. The IP is not currently listed, but it happens from time to time.
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