Select other Welcome Email in Marketing Cloud

I would set up a new Triggered Send definition in the account and then call it manually using either the SOAP or REST API's. Populate the Triggered Send request with the info of the new subscriber you just successfully added. There are a couple ways to do this with ET's APIs.

SOAP http://help.exacttarget.com/en-US/technical_library/web_service_guide/technical_articles/send_triggeredsend_email/

REST https://code.exacttarget.com/api/messaging-messagedefinitionsends-send

Fuel SDK You can also send a triggered email in PHP with the FuelSDK. The Fuel SDK is an abstraction layer on top of the API's listed above. https://code.exacttarget.com/sdks/fuel-sdk-interacting-triggeredsend

There will always be some sort of rendering problems on Outlook, Windows 10 Mail and some lesser known Email clients (like GMX) if you are planning on using standard SFMC Content Builder blocks as they are.

While I haven't checked your code for the Emails, the client of yours needs to understand that by using Content Blocks the Email code is prepared by the SFMC in the back - and it will not always be 100% compatible with all devices / Email clients.

If client wants fully responsive Emails that work perfectly in all browsers/devices/clients then the only way to go is pure HTML/CSS with AMPScript.

With current scenario there are two options:

• You create custom blocks (Free Form or HTML) that are prepared by you that already have all the Outlook specific fixes applied.

• You utilize Content Builder SDK and create fully custom content blocks for the client to use. This is very code heavy option though.

Perhaps useful to read:

• Troubleshoot Microsoft Outlook HTML rendering for Marketing Cloud email
• Marketing Cloud Email alignment problems in Microsoft Outlook
• Content Builder Best Practices and Tips