Would you be okay with something like mail.domain.com?

I only need to send activation email to users. please can i kniw what do you mean by mail.domain.com

Set your txt record on mail.domain.com.

Ok i will check and let u know

I have changed host of my TXT record to mail.mydomain.com and tryied to verify dns records via mailtrap.io. but still verification missing.

What is the actual domain in question?

deeflow.com

There is no TXT record for mail.deeflow.com

But i added it to my domain provider namecheap.

It has not propagated yet then.

Okay i will check after some time

As mentioned by @brody, we can use "mail.yourmaindomain.com" (subdomain) as sending domain for mailtrap. but we need do some changes for our mailtrap DNS records below

1. Domain verification

2. DKIM

3. SPF

4. DMARC

first add mailtrap provided DNS records to your DNS provider advanced dns. since now we are using subdomain, we need to add .mail prefix to the end of "host" value like this

xxxxxx.mail

rwmt1._domainkey.mail (update both DKIM CNAME)

_dmarc.mail

mt-link.mail

then @ (host) value of the TXT SPF record should be replace to "mail".

Finally you need to add 2 MX records to your mail setting DNS records section.

1. Select Custom MX

2. add below 2 MX records

MX Record mail mx1.yourmaindomain.com. 10 Automatic

MX Record mail mx2.yourmaindomain.com. 20 Automatic

and click on save changes.

follow this link for more details
https://simplelogin.io/docs/custom-domain/registrars/namecheap/namecheap-subdomain/

then you can verify your domain from mailtrap.io and will proceed to security check step. They will ask from you why the main domain diff from sending domain. Tell them about this TXT @ record and CNAME @ record conflict. they will verify your subdomain. Thanks Brody