How to configure sent.as for your iOS Device

First, a few words about sent.as:

Fastmail is an email hosting company based in Melbourne, Australia. In addition to its Fastmail-branded services, the company also operates Topicbox, a mailing list service, and Pobox, an email service it acquired in 2015. The company was acquired by Opera Software in 2010 but became independent again in 2013 through a staff buyout. Its servers are located in Bridgewater, New Jersey and Seattle, Washington. Source: https://en.wikipedia.org/wiki/Fastmail

Configuring sent.as:

It is possible that the Mail app will find your parameters itself, and complete setting up your account. Apply these steps first in order to try:

1. Go to Settings > Mail, and click Accounts
2. Tap Add Account > Other and tap Add Mail Account.
3. Enter your name, email address, password and a description for your account.
4. Click Next. The Mail app will try and find the e-mail parameters and complete your account setup. If Mail finds your e-mail settings, click Done to complete your account setup.

In case Mail didn't find your mail parameters, it means that you will have to enter them yourself. You may find the information below useful:

Settings Needed for Receiving Mail with IMAP

Outgoing E-mail Setup

Tap Next, then follow these steps:

1. Select either IMAP or POP If you are not sure which one to choose, contact sent.as
2. Enter the details (You will want to take a look at the table above) for Incoming Mail Server and Outgoing Mail Server. Then click Next
3. If your mail settings are correct, click Save to complete the setup. If your email settings are incorrect, you will be asked to modify them
4. In case you still can't configure the account or save the e-mail settings, contact the e-mail provider.