Replace hard coded value by constant

[vonglasow]

https://github.com/hoaproject/Mail/pull/27/files#diff-b518b20b529a6cf4ff44c6fba63eee4fR273

In this code, we use a lot a hard coded value which represent smtp return code and other stuff. This is not so explicit it could be better to have a constant related.

What do you think?

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[Hywan]

Hello :-),

As said on IRC: “This is just codes, this is hard to associate a name to them”. But we can try by re-reading all the RFCs, thoughts?