<p>Please see <ahref="https://support.google.com/pixelphone/answer/2844832?hl=en">here</a> how you can import certificates into the Android key store.</p>
<p>The use of expired keys, inline encrypted/signed messages and hardware security tokens is not supported.</p>
<p>If you are looking for a free (test) S/MIME certificate, see <ahref="http://kb.mozillazine.org/Getting_an_SMIME_certificate">here</a> for the options. Please be sure to <ahref="https://davidroessli.com/logs/2019/09/free-smime-certificates-in-2019/#update20191219">read this first</a> if you want to request an S/MIME Actalis certificate.</p>