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SMTP recipient's server gives 5.0.0 Service unavailable for some messages but not for others

0

I run a CentOS 7 sendmail SMTP server. For some addresses (a@myclient.com), a virtusertable entry causes my server to forward messages addressed to that address to the client's ISP (charter.net) or gmail. This works for 80-90% of the messages; but for other messages, charter.net or gmail reject the message with DSN 5.0.0 Service unavailable. Note the DSN code is not 4.X.X, deferred, but 5.X.X, rejected. I suspect this happens when the receiving server thinks the message is spam, but it sometimes happens on legitimate emails from, for example, the client's accountant. It seems more likely to happen when there is an attachment to the email.

Does anyone know for sure why this happens? What can I do to reduce the probability of legitmate email being rejected? Might it help to set up SPF/DKIM/DMARC records for myclient.com? For my server's canonical name (which it sends in EHLO records)? Can a server add a DKIM signature to email it is only forwarding?

Thanks for your attention,
Bob

email smtp email-server sendmail

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BlobBlob

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I run a CentOS 7 sendmail SMTP server. For some addresses (a@myclient.com), a virtusertable entry causes my server to forward messages addressed to that address to the client's ISP (charter.net) or gmail. This works for 80-90% of the messages; but for other messages, charter.net or gmail reject the message with DSN 5.0.0 Service unavailable. Note the DSN code is not 4.X.X, deferred, but 5.X.X, rejected. I suspect this happens when the receiving server thinks the message is spam, but it sometimes happens on legitimate emails from, for example, the client's accountant. It seems more likely to happen when there is an attachment to the email.

Does anyone know for sure why this happens? What can I do to reduce the probability of legitmate email being rejected? Might it help to set up SPF/DKIM/DMARC records for myclient.com? For my server's canonical name (which it sends in EHLO records)? Can a server add a DKIM signature to email it is only forwarding?

Thanks for your attention,
Bob

email smtp email-server sendmail

share

asked 1 min ago

BlobBlob

1

New contributor

Blob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

I run a CentOS 7 sendmail SMTP server. For some addresses (a@myclient.com), a virtusertable entry causes my server to forward messages addressed to that address to the client's ISP (charter.net) or gmail. This works for 80-90% of the messages; but for other messages, charter.net or gmail reject the message with DSN 5.0.0 Service unavailable. Note the DSN code is not 4.X.X, deferred, but 5.X.X, rejected. I suspect this happens when the receiving server thinks the message is spam, but it sometimes happens on legitimate emails from, for example, the client's accountant. It seems more likely to happen when there is an attachment to the email.

Does anyone know for sure why this happens? What can I do to reduce the probability of legitmate email being rejected? Might it help to set up SPF/DKIM/DMARC records for myclient.com? For my server's canonical name (which it sends in EHLO records)? Can a server add a DKIM signature to email it is only forwarding?

Thanks for your attention,
Bob

email smtp email-server sendmail

share

asked 1 min ago

BlobBlob

1

New contributor

Blob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

asked 1 min ago

BlobBlob

1

New contributor

Blob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

asked 1 min ago

BlobBlob

1

New contributor

Blob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 1 min ago

BlobBlob

1

New contributor

Blob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 1 min ago

BlobBlob

1