Add email mappings

[simplylizz]

KostyaEsmukov/smtp_to_telegram#28 - like this, but for inboxes instead of from-based mapping.

[smashkins]

+1

[simplylizz]

@smashkins actually it's not that hard, especially taking into account this linked PR. If you want to do it yourself, I don't mind merging & releasing a new version, I just don't need this feature for myself right now, but maybe later.

[smashkins]

@smashkins actually it's not that hard, especially taking into account this linked PR. If you want to do it yourself, I don't mind merging & releasing a new version, I just don't need this feature for myself right now, but maybe later.

I'm not an expert in go. From what I understand, I simply replace from roei3000b PR e.MailFrom.String() with e.RcptTo.String().

It's useful for Synology users who want to send Telegram messages to different channels based on the event triggered by the NAS.
Either way, I'll give it a try, thanks!

[MuratovAS]

I have implemented the email mappings function.
You can find it in my fork: https://github.com/MuratovAS/smtptg