Mail standard says that what is before the @ could be case-sensitive and is under control of the host system, and what is after @ is non case sensitive as under the control of the mail delivery system.

In practice prefix is very rarely considered as case sensitive (I've never encountered any such system), so you can use any casing you want that will work with very very high probability. My official address is something like [email protected] and I always present it as [email protected], I've never had any problem for more than 25 years...

The mail standard says that what is before the @ could be case-sensitive and is under control of the host system, and what is after @ is case insensitive as it is under the control of the mail delivery system.

In practice the prefix is very rarely considered as case sensitive (I've never encountered any such system), so you can use any casing you want and that will work with very very high probability. My official address is something like [email protected] and I always present it as [email protected]; I've never had any problem for more than 25 years...