According to Wikipedia Email addresses can contain comments in parentheses like john.smith(comment)@example.com.

I tried to send out some mails to valid Email addresses using such comments using several Email servers but all of them complained that the email address does not exist (Unknown address error 550-'Requested action not taken: mailbox unavailable' (delivery attempts: 0)>).

Do Email servers not support comments usually and what is the reason for getting this error ?

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    What type of client you have used to generate this email and please add your email server type and related more details Feb 13 '18 at 13:48
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    This is like extended addressing, which needs to be configured server-side for which character separates address from comment area. Note that the default is + but I've noticed issues when sending/receiving via Office365... so on my servers I've changed the character to -
    – ivanivan
    Feb 13 '18 at 14:03
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    @ChamindaBandara: I tried Evolution and the Outlook web interface. Feb 13 '18 at 14:31
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    @ivanivan: Are you sure it's an server-side extension? Comments are part of RFC 5322. Feb 13 '18 at 14:31
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    RFC 5322 section 3.4.1 states that Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec. However, there are 5 entries for this specific statement in the RFC 5322 errata, suggesting the wording or intent is unclear to implementers. I would guess some MTA vendors treat it as "MAY NOT" and simply disallow it. Feb 13 '18 at 20:47

I'm somewhat guessing, but maybe this still helps you: There are two places where the recipient address is used. First is inside the e-mail as a header-field ("To:"), second is in the SMTP dialog when issueing the RCPT command. The latter is usually called the "envelope" recipient.

RFC5322 describes the Internet Message Format (i.e. the header and content of an e-mail message), but not the SMTP protocol. In RFC5321 (which is the SMTP pendant to RFC5322) there is no mention of a comment in an e-mail address.

Thus, chances are that your servers are right, because your client should have removed the comment before using it as an envelope-address. If that had been done, your envelope-sender would be SMTP-friendly for the server to handle and the comment is still inside the e-mail (as a header) for the recipient.

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