电子邮件地址本身对字符大小写不敏感。这意味着实际的电子邮件地址，如 "example@email.com"，并不区分字母的大小写。无论你输入的是大写字母还是小写字母，它仍然会到达同一个电子邮件账户。例如，如果您的电子邮件地址是 "YourName@domain.com"，但有人输入的却是全小写字母，那么它仍然会被正确发送。不过，值得注意的是，"@"符号之前的部分，即本地部分，有时会因主机系统的不同而区分大小写。但实际上，大多数电子邮件系统都将本地部分视为不区分大小写。因此，一般来说，在发送或接收电子邮件时，不必担心电子邮件地址的大小写敏感性。
电子邮件地址的用户名，不建议使用的字符： & = _ ‘ - + 。因为有些邮件服务器不一定支持这些特殊字符。还有一些英文之外的字母，比如拉丁字符，也不一定支持。
有些电子邮件地址名可以使用dot字符。比如zhang.san@site.com。
Email addresses themselves are not character sensitive. This means that the actual email address, such as "example@email.com", doesn't care about the case of the letters. Whether you type it in uppercase or lowercase, it will still reach the same email account. For example, if your email address is "YourName@domain.com" but someone enters it in all lowercase letters, it will still be delivered correctly. However, it's worth noting that the part before the "@" symbol, referred to as the local part, can sometimes be case-sensitive depending on the host system. But in practical terms, most email systems treat the local part as case-insensitive. So, in general, you don't need to worry about the case sensitivity of email addresses when sending or receiving emails.
扩展阅读：
1，Email地址区分大小写吗
Are Email Addresses Case Sensitive? And Other Email Best Practices | Mailchimp
