I believe the single quote violates the RFC.

RFC 2821 (www.rfc-editor.org/rfc/rfc2821.txt) states that the RCPT TO command must be specified as follows:

RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>

   The first or only argument to this command includes a forward-path
   (normally a mailbox and domain, always surrounded by "<" and ">"
   brackets) identifying one recipient. 

The question then becomes "how do you define a "forward-path". That is specified in the following section of the same RFC. Here's the relevant part. Please note that the key piece is the one at the bottom colored in purple, as it ultimately defines what the smaller piece of text can be. It states that the text can be delimited by a "DQUOTE" which is a double-quote, *NOT* by a single quote as in the case you saw.

4.1.2 Command Argument Syntax

   The syntax of the argument fields of the above commands (using the
   syntax specified in [8] where applicable) is given below.  Some of
   the productions given below are used only in conjunction with source
   routes as described in appendix C.  Terminals not defined in this
   document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in
   the "core" syntax [8 (section 6)] or in the message format syntax
   [32].

      Reverse-path = Path
      Forward-path = Path
      Path = "<" [ A-d-l ":" ] Mailbox ">"
      A-d-l = At-domain *( "," A-d-l )
            ; Note that this form, the so-called "source route",
            ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
            ; ignored.
      At-domain = "@" domain
      Mail-parameters = esmtp-param *(SP esmtp-param)
      Rcpt-parameters = esmtp-param *(SP esmtp-param)
      esmtp-param     = esmtp-keyword ["=" esmtp-value]
      esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
      esmtp-value     = 1*(%d33-60 / %d62-127)
            ; any CHAR excluding "=", SP, and control characters
      Keyword  = Ldh-str
      Argument = Atom
      Domain = (sub-domain 1*("." sub-domain)) / address-literal
      sub-domain = Let-dig [Ldh-str]

      address-literal = "[" IPv4-address-literal /
                            IPv6-address-literal /
                            General-address-literal "]"
            ; See section 4.1.3

      Mailbox = Local-part "@" Domain

      Local-part = Dot-string / Quoted-string
            ; MAY be case-sensitive

      Dot-string = Atom *("." Atom)

      Atom = 1*atext

      Quoted-string = DQUOTE *qcontent DQUOTE

      String = Atom / Quoted-string