I've been getting errors of "hostname suspect" when sending to Optimum
Online
At the envelope stage, there are really on 4 fields that an MX knows about
1. IP of the SMTP client has a single PTR domain name which MUST
have, as best practice, a matching A record.
2. SMTP client's HELO domain name.
3. MAIL FROM: envelope sender
4. RCPT TO: envelope recipient
"hostname suspect" would probably be the HELO hostname with:
1. illegal characters (illegal in DNS hostnames)
2. an IP address not in domain literal syntax [ip.ad.re.ss]
3. hostname not fully qualified
4. fully qualified hostname that has no A or MX records
HELO hostnames have always been one of the best places to identify spam.
Len
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