Add SMTP_FROM environment variable for flexible sender addresses

[Chasnechok]

Description

• Problem: Currently, emails are sent from the SMTP_USER address, which is also used for authentication. This limits the ability to use email aliases.
• Solution: Introduced a new environment variable, SMTP_FROM, to set the sender's email address. This allows emails to be sent from different addresses (aliases) without the need to change authentication credentials. If SMTP_FROM is not configured, the system falls back to using SMTP_USER for compatibility.

Example Use Case:

[email protected]
[email protected] 

[danenania]

Thanks! It will be included in the next server release (or will be included now if building from source).