A formal writing style is not necessarily “better” than an informal style; each style serves a different purpose and the language you use in each situation must be appropriate in style and tone.
It is true that business environments tend to be more casual nowadays than they used to be in the past but writing for professional purposes is likely to require a formal style.
Follow these basic rules of formal writing and cause a good impression:
1. Write all verbs in full. Do not use contracted forms like don’t or can’t, but do not and cannot. Write they have rather than they’ve, and they will rather than they’ll.
2. Do not use abbreviations such as asap (for as soon as possible).
3. Limit the use of passive voice. However, there are some situations where passive verb forms are preferred to active forms. For example, active verb forms in the first person singular are not considered appropriate in formal or academic writing. Write ‘The document will be e-mailed to you as soon as possible’ instead of ‘I will e-mail you the document asap’.
4. Watch your vocabulary. Certain words are considered informal. Examples are: fix, start, OK, thanks, etc. Avoid them in formal writing. Instead use words like repair (for fix), commence (for start or begin), in order / all right (for OK) and thank you (for thanks). Write incidentally instead of by the way.
5. Avoid informal intensifiers like really and so. Instead use more formal ones such as extremely, highly, entirely, remarkably, etc.
6. Do not leave out words. Write ‘I hope to see you soon’ instead of ‘Hope to see you soon’; “I will be looking forward to hearing from you”, rather than “Looking forward to hearing from you.”
7. Avoid phrasal verbs whenever it is possible to express the same idea using non-phrasal verb forms. For example, write cancel a meeting, rather than call it off; reject an application, instead of turn it down; postpone, better than put off; visit, rather than come over; arrive, instead of turn up.
Finally, note that e-mails tend to lend themselves to a less formal style than paper-based communications, but you should still avoid the use of "text talk".