When there is white space present in the user name("Abuse Filter", "Wiki Admin") for an user then it will break email headers if not contained in double quotes. The safest way is to just always wrap the name in double quotes and not selectively.
What software requires it to be quoted? I just created "Lego test account number 1" and sent email to and from that account with no issues. The raw headers looked like To: Lego test account number 1 <email> and Reply-To: Lego test account number 1 <email>. Both displayed fine in Thunderbird, and presumably went through Wikimedia's mail setup properly.
I will fix this patch this week.
It was an issue that came up with AWS Simple Email Service that was made worse with an older version of sendmail. The headers were:
To: User Space <email@example.com>
Reply-To: Help <firstname.lastname@example.org>
It would end up parsing the To: header as: "To: User Space email@example.com Reply-To Help <firstname.lastname@example.org>"
Obviously that should not happen, but after spending too much time reading the RFCs I eventually settled on there be implementation issues in software out of my control. The CRLF present in the headers should have noted the end of the To: header to the parser. I went with the always quote the name in the email approach since it fixed the issue in all cases.
If I remember correctly this is the relevant RFC section: https://tools.ietf.org/html/rfc5322#section-3.2.1