Steps to replicate the issue (include links if applicable):

• Open an account on he.wikipedia.org, with the username composed of at least a Hebrew letter (any character in the range א-ת) and a comma character (,)
• Provide a valid email address during the account set-up process (e.g. an address you've already confirmed)
• Upon completion, check the inbox of the provided address for a welcome email. Also, from the user preferences, try to send a confirmation email

What happens?:

No confirmation email arrives. When trying to resend from the user preferences, no email arrives, and the following error message (or a translation thereof) is displayed:

Wikipedia could not send your confirmation mail. Please check your e-mail address for invalid characters.
Mailer returned: Unknown error in PHP's mail() function

What should have happened instead?:

A confirmation email should have been sent. Additionally, the error message is misleading, since there's no problem with the email address.

If the user changes their username by only removing the comma, or by replacing it with a period (.), the issue disappears; most hewiki users whose usernames are in Hebrew don't encounter this issue.

Also, a username with English letters and a comma (and a space) does not have this issue; a confirmation mail was successfully sent when I renamed the test account into an English version (with a comma).

It seems like the concurrence of both a comma and Hebrew letters causes this issue to manifest itself. I didn't test other punctuation marks (other than period).

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

Not browser-related, as far as we could tell.

This bug may be related to T32260 (specifically, T32260#357289 gave me the hint that the issue may be related to the username and not to the email address, despite what the error message says).