Custom sender address

Expected behavior

In some mail clients like thunderbird, evolution or kmail you can enter a custom sender address.
I think the feature is really convenient when using the same account for multiple issues (private, work,…)
This issue is similar to #301 but different ;)

I want to be able to modify the sender field not only to set the default identity of each account but also enter new temporary ones. Like sending one mail from me@example.com and the next mail from work@example.com.

Of course this feature has to be enabled in the settings of an account, because not all mail provider support this feature.

I found the feature with the aliases but I prefer temporary aliases, because I have a custom address for each client and it would take to long to look for the required identity

Actual behavior

I have to use the account configured without any modifications

Mail app

Mail app version: (see apps admin page)
0.7.7
Mailserver or service: (e.g. Outlook, Yahoo, Gmail, Exchange,...)
mailbox.org
Transport security - IMAP: (None, SSL, TLS, STARTTLS)
ssl
Transport security - SMTP: (None, SSL, TLS, STARTTLS)
ssl
Number of accounts:
1

The missing information is not required for this issue, if needed later I will post them.

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The issue names no files or tests. Start by tracing the account settings and SMTP sending flow, then determine how temporary sender addresses should be enabled and validated; done means a user can send successive messages from different temporary addresses without changing the account identity.