How can I add a second sender address with different SMTP server in iOS?
Configuring additional outgoing mail servers
from the Apple support websites has detail instructions:

Go to Settings → Mail, Contacts, Calendars.

Tap the Mail account for which you would like to change the settings.
If your account is configured to sync Notes or other items, you may need to tap Account Info on the following screen as well.

On the Account Info screen, locate the Outgoing Mail Server section and tap the existing SMTP entry.

A list of SMTP servers will appear, with the Primary Server and then Other SMTP Servers. Go to Other SMTP servers → Add Server....


Enter the additional SMTP server information. Host Name is required.

Tap Save.