Surprise, the local part of email addresses, i.e. the part in front of the @-character, is per RFC case-sensitive. Do not mess with the local part.

Relevant when sending out challenge mails, account validation mails, password reset mails, issuing S/MIME certificates, etc.

Luckily a lot of the internet infra just ignores the case, but Apple Mail does not.

#RFC5321 #RFC #email #SMTP #MTA #SMIME