After a new casting company or person requesting your services, is it ok to email them a thank you? If so, do they think it is kissing up or being respectful?
Technology has changed every aspect of our industry so lets start with the question Who Hires Actors for jobs today.
Reply Casting Directors, Talent Agency's, Advertising Agencies, Production Companies, Theaters, Photographers ETC
If They Hire you for a job is OK to email them a thank you?
Traditionally MAILING (Not Emailing) a Thank you to the Employer after you completed a job they hired you for was almost Expected.
This, of course was so long ago My Grand Nephew (Sisters Grandson) Jackson (age 2) was not born yet
Employers are emailed all kinds of stuff, everyday and Most thank you notes are thought of as old hat.
My personal feeling is that Thank You Notes are Still appropriate if the job is big enough but should never be emailed.

They are not appropriate for Extra Jobs (unless very high visible and high paying) or for just another Day Player Role (but ok if it is the 1st Day Player Role)

If you are sending 1 out- It needs to be a card you bought, note kept short and personal (with no Advertising for future work) and mailed with a stamp on the envelope.