what I would do is drop thae email string and do a people picker and have the ability to select more than person. If your bringing back a type person you have more flexibilty and it will not have all the crazy html that SharePoint adds to email Strings.

Can you confirm that the text column is set as 'plain text' and not as rich text?

Previously, with this kind of issue, I have used a people picker column. Set the display format for the column to email. And I would add a small on-change Nintex workflow to copy the contents as text to a plain text column.

Hi Shanan,

Not a problem to do this.  My guess is that you've got an external email address in your list so SharePoint online/o365 is complaining about this because it doesn't like sending outside the organisation.  The magic option to tick in the send email action is the "External email and attachment" option.  That will allow you to send both inside and outside the organisation.

Naturally your to/cc/bcc fields will reference a single line of text field that you have delimited using semi-colons.

Cheers,

Chris

Thanks ‌ I've set the field as 'Multiple Lines' then set it as 'plain text' underneath that and it seems to work.

‌ I've set it as 'External' as default since the emails would be both internal & external. Thanks for the help!

This is a bit of an old thread, but I had this same issue and Chris Ben's solution does work, HOWEVER, it requires that the semicolon delimited field have NO SPACES in it. 

examples:

FAILS: name@address.com; name2@anotheraddress.com
WORKS: name@address.com;name2@anotheraddress.com‍‍

Spent way to many hours banging my head against the wall before I discovered this, so I'm sharing to help others out. In my case, I was using a "single line of text" field.