In my opinion using personal email address for official company communications is ridiculous, however it all comes down to the company policy and requirements towards mailing. Companies usually have acceptable use policy and ethics codes that speak about not using personal email for company related matters. In addition corporate emailing is usually company managed and security policies are applied to it - not to mention the auditing and logging features, but I guess it all comes down to the company and its reuqirements towards mailing. Security-wise - I'd say anything outside the predefined and pre-approved way of mailing is wrong - personal mailing of business matters is nonsense and introduces a variety of risks.
Personal email should not be used for business purposes and business email should not be used to sign up to non-business services.
Using personal email addresses for business purposes can blur the lines of privacy also. Since you used your personal email address for business purposes does that mean in an investigation your personal email account can be subpoenaed? Can investigators seize your device when you leave the company to make sure there is no company data on your device(s)? Is an employee exfiltrating sensitive company data through their personal email (think Edward Snowden type stuff)? If they receive work related emails on their personal account on the weekend does that mean that they are on the clock and should expect to be compensated for working that time?
You can see that allowing, or heaven forbid - requiring, an employee to use their personal email address for business use can have consequences you may not have planned for. This topic is heavily discussed in BYOD scenarios if you need more resources as to what you would be getting in to.