In Thunderbird for desktop, in a given account, each identity not only has settings for mail address and signature, but it is also possible to choose an alternate SMTP server to be used.
This is more important than ever when working with mail aliases, due to increased SPF and DKIM checks.
Thunderbird for Android currently only allows one SMTP server per account, not one per identity in an account. This makes it impossible to send mail for an identity that is actually a mail alias.