Should domain names and email addresses always be lowercase to make sure you get where you want to go? Or is this completely irrelevant? For example, Google's mail servers have curious settings with interesting effects, which I will briefly introduce in the following article.
Google, which recently started offering @gmail.com addresses to German users as well, ignores dots before the spider monkey and the spelling is case-insensitive, so the following email addresses are completely equivalent:
The standard RFC 2821 specifies how this should normally be handled:
The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.
This means that the domain name behind the spider monkey is case-insensitive and the local part before the spider monkey is case-sensitive. In most cases, however, mail servers are very tolerant. I recommend always writing both email addresses and domain names in lower case.
A small aside: You can also add an additional address directly in front of the @, which will be ignored during delivery (and can therefore be used, for example, for rule-based filtering of emails):