This often happens because of formatting issues of the email client. You might have copied your email template from MS-word, some website or from an old email. When you copy this data from one source and paste it into another, a lot of hidden formatting gets added into your template. This new formatting might not support in all email clients and messes up the email.
In order to create your template, we strongly recommend first passing it through a plain-text editor. Copy your text and then paste it into the plain-text editor (notepad) to strip out all the formatting. Then, copy it and paste it into your template. This would solve the issue. Also, test your emails before sending.
Solution: In order to create your template by copying data, we strongly recommend first passing it through a plain-text editor. Copy your text and then paste it into the plain-text editor to strip out all the formatting. Then, copy it out of the plain-text editor and paste it into your template.