Feeling tired and frustrated with the default 'Sent from My iPhone / Sent from My iPad' signature of the Mail app on iOS, and you want to replace it with something like a fully functional HTML signature with clickable URLs and various rich styles?

Webmail makes it easy to create an HTML signature with features like bold and italic text, changing font sizes, and creating links. Design your signature on webmail, then send an email to your iOS device, and that copy will be used to create the signature on the iOS device.
Create HTML Signatures on iOS.
Compose a new email to send to the email address set up on your iPhone, and use the webmail tools to create a signature that reflects your preferences.

Afterward, send the email to your iPhone > on your iPhone, open the email > tap and hold on the email content > Select > Choose the signature you just designed > Copy.
Go to Settings > “Mail, Contacts, Calendars” > Select “Signature”. Tap and hold, then select 'Select All' > “Paste” to paste the signature you created from webmail.

After that, exit Settings and resend the email to yourself to ensure the signature has been successfully updated. You can apply this trick on all iOS versions, regardless of the webmail you use to design the signature, be it Hotmail, Gmail, Yahoo Mail, and more.
Source: osxdaily.com
