Right, and trim is for the beginning and end. FUTO Keyboard will put spaces after periods, so it'll frequently try to enter something like first. Last @domain.Com. The casing isn't an issue because emails (and all URLs) should be treated as case insensitive.

I wouldn't expect a site to remove all whitespace, only leading and talking trailing whitespace, and then present an error if the email address is obviously invalid. There are libraries for this, and I think a simple regex would also be sufficient to catch most issues (search online for a vetted one).

Spaces are technically allowed before the @, provided you wrap it in quotes. That's incredibly rare and validating that is a bit of a pain, so I'd stick with making it an error instead of silently stripping what could be a valid, but unsupported, email address, since that would cause more confusion than an error.