Feature/mail queue

[michaelernst]

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For documenting purposes and potential debugging it is quite useful to have the mail queue ID at hand.

This feature adds storing the mail queue ID of the last send() call.

It also resets members used to store server responses. In case a sender instance is used multiple times they contain information of the last call.

[dantti]

also you forgot to add this to async Server class

[michaelernst]

also you forgot to add this to async Server class

I have not used the async class so far. Where would you put this?

[dantti]

https://github.com/cutelyst/simple-mail/blob/master/src/serverreply.cpp#L44

[dantti]

Ok, nvm I finish the changes

[dantti]

I'm closing this as I removed the blocking code, if you want this in 2.0 branch please rebase and create a new PR. Thanks, sorry for delay