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.