JMAP as replacement of IMAP

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Currently in Delta Chat attachments are base64-encoded using MIME Content-Transfer-Encoding: base64, but if OpenPGP is used then the whole payload is signed, compressed and encrypted, so compression negates the effect of base64-encoding. Afterwards base64 is applied again, but as ASCII armor rather than MIME encoding, this is unavoidable as ASCII armor is required by RFC 3156: MIME Security with OpenPGP

For unencrypted messages sending binary data without any encoding is theoretically possible, but in practice we never know if receiving server can accept binary 8-bit emails:

Integrated compressor

OpenPGP already compresses the message losslessly before encryption, but after encryption the message is base64-encoded. We can hardly do anything about it, because other encodings for OpenPGP are not standardized. There is a binary MIME content-transfer-encoding, but it is not widely supported and can’t be used to re-encode OpenPGP messages: OpenPGP messages are not using base64 Content-Transfer-Encoding, but ASCII-Armoring instead. OpenPGP message is basically a text message starting with ----…

So there is no easy way to avoid base64 overhead when sending mails.

However when the mail is finally delivered to the server, we can get rid of base64 overhead. JMAP servers can decode MIME base64 encoding back when the mail is finally delivered, but IMAP COMPRESS or TLS compression can achieve the same effect and also work for OpenPGP ASCII armor.