Example 1
Typing e by hand does not create an expression meaning the base of the natural logarithm ($ExponentialE;\approx 2.718231828$ ). Instead, the e is simply a variable.
Solution 1  Use the command/symbol completion
Use command/symbol completion to enter the exponential constant $\ⅇ$. Start by typing e, then press Esc. Select Exponential 'e' from the popup list.
${\ⅇ}$
 (2.2) 
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${\ⅇ}^{x}$

${{\ⅇ}}^{{x}}$
 (2.3) 
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$\mathrm{plot}\left({\ⅇ}^{x}\,x\right)$

Solution 2  Use the Palettes
The Expression palette provide templates for entering mathematical expressions. It's useful when you want to write a power of $\ⅇ$. Alternatively, the Common Symbols palette has an entry for $\ⅇ$ which you can incorporate into a larger expression.
To use the Expression palette, place your cursor in your document, then click the
template. The placeholder is highlighted. Type x. Press enter to execute.
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${\ⅇ}^{x}$

${{\ⅇ}}^{{x}}$
 (2.4) 
Solution 3  The exp Function
You can also use the exp function:
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$\mathrm{exp}\left(x\right)$

${{\ⅇ}}^{{x}}$
 (2.5) 
See the exp help page for additional information.
Example 2  Prevent this warning from appearing
Suppose do you want to use e as a name (a variable), and did not intend it to mean the exponential e.
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$\left\{{e}^{3}\=d\+2\,e\+d\=8\right\}$

$\left\{{{e}}^{{3}}{=}{d}{+}{2}{\,}{e}{+}{d}{=}{8}\right\}$
 (2.6) 
Solution
You can control whether parse warning messages appear via Typesetting:Settings.
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$\mathrm{Typesetting}:\mathrm{Settings}\left(\mathrm{parserwarnings}\=\mathrm{false}\right)\:$

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$\left\{{e}^{3}\=d\+2\,e\+d\=8\right\}$

$\left\{{{e}}^{{3}}{=}{d}{+}{2}{\,}{e}{+}{d}{=}{8}\right\}$
 (2.7) 
Now you do not see the warning message.