A method, executed by a security gateway , for processing a request that a client device acquires an email message from an email server, the method comprising the steps of: receiving a first request sent by the client device, wherein the first request is used to acquire a portion of the email message, and the portion of the email message does not comprise an email header of the email message; converting the first request into a first converted request used to acquire the entire email message , wherein the first request sent by the client device and the first converted request are formatted according to Internet Message Access Protocol 4, IMAP4; sending the first converted request to the email server; receiving the email message returned by the email server, wherein the email message is an email message in encoded form; analyzing the email message to obtain encoding information of the email message, wherein the encoding information includes Content-Transfer-Encoding information, and the Content-Transfer-Encoding information is obtained by analyzing an Multipurpose Internet Mail Extensions, MIME, information header or an MIME segment header of the email message ; decoding the email message according to the encoding information, so as to obtain the portion, in decoded form, of the email message; performing security processing on the portion, in decoded form, of the email message, so as to determine whether the portion of the email message is secure; and forwarding the portion of the email message to the client device if the portion of the email message is determined to be secure; or refusing to forward the portion of the email message to the client device if the email message is determined to be an insecure email. receiving a first request sent by the client device, wherein the first request is used to acquire a portion of the email message, and the portion of the email message does not comprise an email header of the email message; converting the first request into a first converted request used to acquire the entire email message , wherein the first request sent by the client device and the first converted request are formatted according to Internet Message Access Protocol 4, IMAP4; sending the first converted request to the email server; receiving the email message returned by the email server, wherein the email message is an email message in encoded form; analyzing the email message to obtain encoding information of the email message, wherein the encoding information includes Content-Transfer-Encoding information, and the Content-Transfer-Encoding information is obtained by analyzing an Multipurpose Internet Mail Extensions, MIME, information header or an MIME segment header of the email message ; decoding the email message according to the encoding information, so as to obtain the portion, in decoded form, of the email message; performing security processing on the portion, in decoded form, of the email message, so as to determine whether the portion of the email message is secure; and forwarding the portion of the email message to the client device if the portion of the email message is determined to be secure; or refusing to forward the portion of the email message to the client device if the email message is determined to be an insecure email. <EOS>