The "From" address used when you send an email depends on the setting in the email program from which you send the message. If you send messages from different devices, the "From" address might be different. Follow the directions below to ensure your "Sender identity" is consistent and set to the value you want.
If you are using Roundcube webmail, follow these instructions.
When you login for the first time, you are prompted to enter your identity information. That's the easiest way to do it!
If you didn't do it when you first logged in, no worries, you can do it at any time.
Also, you can add more then one identity.
With the Horde/IMP webmail program, you can control how your email messages are sent by setting up your Default Identity:
With Thunderbird, you can create both separate "accounts" and also you can have one or more identities per account.
You can access your Account settings either:
Once you have opened your Account Settings dialog, click on the Account name you want to add or change the identity for. You should see a form with your default identity, along with a "Manage Identities..." button add additional ones.