How to write an email to a professor to inquire about a visiting researcher position? I am currently a postdoc and plan to apply for a postdoctoral fellowship as the funding to be a visiting researcher in other institutes. However, to be eligible for applying the fellowship, I would need an invitation letter from the department or institute. I have written about 30 emails from 2 months ago but haven't received any reply. I know that generally postdoc researchers are not so prominent or influential in the academic world so that many professors would not easily accept such requests. And I also understand that it would be more difficult if I do not know the professors personally through conferences or other social networks. But there should be at least a reply of refuse due to any kinds of reasons. 

I wonder if there is any serious problem with my email. I would be very appreciated if anyone could kindly find any problems in it. 


  Dear Prof. XYZ,
  
  I am writing to inquire about the opportunity of serving as a visiting researcher in your lab next year (Jan, 2020−Dec, 2020). The funding will be totally covered by the XYZ.
  
  I am currently a postdoctoral fellow focusing on research associated with human health risk assessment of environmental contaminants at XYZ. My major is nanotoxicology and environmental risk assessment; my minor is physiologically-based pharmacokinetic/pharmacodynamic modeling. I have enormous interests in your research after reading your articles regarding toxicities of TiO2 NPs.
  
  I have written a research proposal regarding pharmacokinetic and human health risk assessment of food-grade TiO2 NPs as attached in this email. It would be my great honor and appreciation if you could find time to read this proposal at your convenience and I hope my background in mathematical modeling and experimentation could help me to have a valuable contribution
  I have attached my CV and representative research to this email to provide more information about myself and my academic records. I have also provided the document of the fellowship for visiting researchers as the reference. There will be no expenses required during my visiting except for an invitation letter after your valuable permission to allow me to be a visiting researcher in your research team.
  
  If you require additional information, I would be pleased to provide it at any time. I am eagerly waiting for your kind reply and greatly appreciate your attention and valuable time to read this email.
  
  Yours respectfully,
  
  XYZ, Ph.
I think your email is fine! I'll speak from my experience doing several research internships (I haven't completed my PhD, therefore no postdoc). It obviously depends a lot on your field (ie, your field might be different than mine), but even though I had previous experience and publications, I sent about 100 emails before I got my first phone interview. 

If you have the time to change your email, I might soften the tone a little. You do come off as very eager. For example, I might change the following to: 


  "It would be my great honor and appreciation if you could find time to read this proposal at your convenience and I hope my background in mathematical modeling and experimentation could help me to have a valuable contribution." 


To something more like: 


  "My background in mathematical modeling and experimentation makes me a strong candidate to successfully [work in your lab/collaborate/publish papers/etc]. 


I suggest this because:


saying it would be a "great honor and appreciation" could be interpreted as brown-nosing/sucking up. In America, people are not usually this humble, nor do we usually speak this honorifically about others (but this could depend on your field/where you are applying). 
In America, it is very acceptable to state why you are a strong candidate. Saying things like "I hope I will be successful at..." or "I believe I can..." can make potential bosses feel like you're not sure if you will be successful. Instead, I'd recommend saying "I can..." or "I will..." or "I am..."