Email verification is all about non-return email deliverability by avoiding auto responders that can hurt your sender reputation. Valid emails without a return-path are favorable with email administrators and email service providers. The more emails you send, the more likely you will encounter bounces (hard or soft), vacation emails, challenge responses, replies from list servers or feedback reports. Here is email list verification process explained:

1. Email List Verification Process Explained
My last article was all about email list validation. Is there a difference between
email validation and email verification? Yes there is. Email validation fixes format
issues and syntax errors while email verification reports if an email is valid (does
the mailbox exist) and if not, lists the reason for the return path. Email validation
fixes and/or removes duplicates, disposables, departmentals, botclickers, false
positives, slow responses, forwarders, format errors, unicode characters and in
some cases, checks the mail exchange record for numerous warning signs.
Email verification is all about non-return email deliverability by avoiding auto
responders that can hurt your sender reputation. Valid emails without a return-path
are favorable with email administrators and email service providers. The more

2. emails you send, the more likely you will encounter bounces (hard or soft),
vacation emails, challenge responses, replies from list servers or feedback reports.
An email verification service will use these steps in order to achieve true
verification (not in order and not all verification companies do these step by step):
 Mail advertisements to your list. You get the best results when you send a direct
SMTP handshake with content. This could agitate your subscribers because you are
not in control of the content being sent.
 Appending database. A collection of emails with the last known deliverability date
which is updated often.
 Check social media email addresses. Social Media email addresses tend to be the
unique email address of the holder. Some verifiers have a list of Facebook or
Twitter accounts and append to that.
 Send a blank email via SMTP (simulate a direct message delivery) to authenticate
the user.
 Establish a connection to the mail server via an SMTP protocol (based on TCP). By
using connection-oriented dispatch service packaging to establish the connection
with hello messages, the sender’s address is transmitted and the receiving server
submits the message from this address to be accepted. Then the addressee’s address
is transmitted.
 Emulate a second message dispatch. No message is actually sent during the email
availability check. This process establishes the connection with the mail server,
sends a hello message, transmits the sender’s address (options/sender) to pretend
there is a message and then transmits the subscriber’s mailbox address (an email
address being checked). As soon as the receiving server confirms or denies the
requested mailbox availability, the email verification tool disconnects.
 DNS Response times from two locations (Timeouts). If the verifier does not receive
a response from a DNS server during a specified period of time (1 – 2 seconds),
verifiers will make new attempts using another DNS gateway.

3.  Send through different ports in case of roaming or receiver scenarios. In example, if
port 25 is intercepted, then tunnel to another transparent proxy or through port 587
and/or other options.
 Enhanced mail system status codes check. Success (valid email) specifies that the
DNS is reporting a positive delivery status. Detailed status codes may provide
notification of transformations required for delivery checks.
 A persistent transient failure is when the message sent is valid, but persistence of
some temporary condition has caused abandonment or delay of attempts to send the
message. If the code accompanies a delivery failure report, sending in the future
may be successful.
 A permanent failure is when the email is not likely to be resolved by resending the
message in the current form. Some change to the message or the destination must be
made for successful verification.
 Querying the mail exchange record. Bad MX emails have slow response times, 4xx
errors, full load distribution, the mail exchange isn’t configured correctly, incorrect
port numbers, has no backup server, is parked or the website doesn’t follow typical
mail exchange setup standards.
Here are the reporting parameters of email verification after the above steps have
been completed by an email list cleaning service (not in order and not all
verification companies do these step by step):
Valid 250
The email passed all verification questions and is considered a true deliverable.
Hard Bounce 550
The email server was temporarily unable to deliver your message to the
subscriber’s email address because the subscriber’s address does not exist.
General Bounce

4. The email server could not deliver your message, but the exchange could not
determine a specific reason for the bounce. The bounce message from the
subscriber’s server was widespread.
Soft Bounce
The email server was temporarily unable to deliver your message to the
subscriber’s email address because of Internet outage, interruption and/or
connection timeout issues.
Soft Bounce DNS Failure
The email server is temporarily unable to deliver your message to the subscriber’s
email address because of a DNS connectivity issue or incorrect configuration.
Soft Bounce Mailbox Full
The email server is temporarily unable to deliver your message to the subscriber
because the subscriber’s email box is full.
Soft Bounce Message Size Too Large
The email server could not deliver your message to the subscriber because the
message size is too large.
Transient Bounce
The email server temporarily cannot deliver your message, but it is still trying.
Slow Response
The email server temporarily cannot deliver your message, but it is still trying.
Mail Block Relay Denied 551
Indicates that the subscriber’s email server is blocking email from our email server
whether for spam or other reasons.
General Mail Block 550
Indicates that the subscriber’s email server is blocking email from our email server
whether for spam or other reasons.

5. Mail Block Known Spammer
Indicates that the subscriber’s email server is blocking your email because it
believes you are a spammer.
Mail Block Spam Detected 550
Indicates that the subscriber’s email server is blocking your email because the
message appears to have content that looks like spam.
Auto Reply
An automatic response from the subscriber sent to your server, which could be out
of office or a vacation message.
Forwarder
Report back any disconnected user, roaming or greeting card scenario (for
example, a user can instruct Gmail to forward messages to a different email
address. The sender is not necessarily aware of that) called “Forwarder”.
Bounce With No Email Address (BN)
Determined that the message was a bounce, but it could not resolve the original
email recipient email address. False positive.
Mail Block Attachment Detected (MBAD)
Indicates that the subscriber’s email server is blocking email from your email
server because the email message contained an attachment.
Mail Block – Relay Denied (MBRD) 551
Indicates that the subscriber’s email server is blocking email from your email
server.
Challenge response spam filtering
Spam filter that automatically sends a reply with a challenge to the sender to
perform some action to assure delivery of the original message.
List Server Replies (SMTP Replies)

6. A program that handles subscription requests for a mailing list and distributes new
messages, newsletters, or other postings from the list’s members. (A list server
should not be confused with a mail server, which handles incoming and outgoing
e-mail for Internet users.)
Feedback Reports
A mailbox provider forwards the complaints originating from their users to the
sender’s organizations.
False Positive 550
User account is not there, or they turned on/off a feature that allows for non-listed
users to pass through.
Email verification companies will use their own code to determine the reporting
above. At times, verification cannot get responses from the subscriber’s email
server due to mail server configurations. Administrators make changes to their
mail servers daily as they monitor traffic from the same source or ranges. They set
alerts for anything suspicious especially when a range of IP’s are asking questions
without sending responses. The email verification process is a delicate and
expensive procedure if done right. In this case, you definitely get what you pay for.